docs(http_api): adds docs for alerting in http api

docs/sources/http_api/alerting.md

++++
+title = "Alerting HTTP API "
+description = "Grafana Alerting HTTP API"
+keywords = ["grafana", "http", "documentation", "api", "alerting"]
+aliases = ["/http_api/alerting/"]
+type = "docs"
+[menu.docs]
+name = "Alerting"
+parent = "http_api"
++++
+
+
+# Alerting API
+
+You can use the Alerting API to get information about alerts and their states but this API cannot be used to modify the alert. 
+To create new alerts or modify them you need to update the dashboard json that contains the alerts. 
+
+This API can also be used to create, update and delete alert notifications.
+
+## Get alerts
+
+`GET /api/alerts/`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    [
+      {
+        "id": 1,
+        "dashboardId": 1,
+        "panelId": 1,
+        "name": "fire place sensor",
+        "message": "Someone is trying to break in through the fire place",
+        "state": "alerting",
+        "newStateDate": "2016-12-25",
+        "executionError": "",
+        "dashboardUri": "http://grafana.com/dashboard/db/sensors"
+      }
+    ]
+
+## Get one alert
+
+`GET /api/alerts/:id`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    {
+      "id": 1,
+      "dashboardId": 1,
+      "panelId": 1,
+      "name": "fire place sensor",
+      "message": "Someone is trying to break in through the fire place",
+      "state": "alerting",
+      "newStateDate": "2016-12-25",
+      "executionError": "",
+      "dashboardUri": "http://grafana.com/dashboard/db/sensors"
+    }
+
+
+## Pause alert
+
+`POST /api/alerts/:id/pause`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+    {
+      "alertId": 1,
+      "paused: true
+    }
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    {
+      "alertId": 1,
+      "state":   "Paused",
+      "message": "alert paused"
+    }
+
+## Get alert notifications
+
+`GET /api/alert-notifications`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    
+    {
+      "id": 1,
+      "name": "Team A",
+      "type": "email",
+      "isDefault": true,
+      "created": "2017-01-01 12:45",
+      "updated": "2017-01-01 12:45"
+    }
+
+## Create alert notification
+
+`POST /api/alerts-notifications`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+    {
+      "name": "new alert notification",  //Required
+      "type":  "email", //Required
+      "isDefault": false,
+      "settings": { 
+        "addresses: "carl@grafana.com;dev@grafana.com"
+      }
+    }
+    
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    {
+      "id": 1, 
+      "name": "new alert notification",
+      "type": "email",
+      "isDefault": false,
+      "settings": { addresses: "carl@grafana.com;dev@grafana.com"} }
+      "created": "2017-01-01 12:34", 
+      "updated": "2017-01-01 12:34"
+    }
+
+## Update alert notification
+
+`PUT /api/alerts-notifications/1`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+    {
+      "id": 1,
+      "name": "new alert notification",  //Required
+      "type":  "email", //Required
+      "isDefault": false,
+      "settings": { 
+        "addresses: "carl@grafana.com;dev@grafana.com"
+      }
+    }
+    
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    {
+      "id": 1, 
+      "name": "new alert notification",
+      "type": "email",
+      "isDefault": false,
+      "settings": { addresses: "carl@grafana.com;dev@grafana.com"} }
+      "created": "2017-01-01 12:34", 
+      "updated": "2017-01-01 12:34"
+    }
+
+## Delete alert notification
+
+`DELETE /api/alerts-notifications/1`
+
+**Example Request**:
+
+    GET /api/org HTTP/1.1
+    Accept: application/json
+    Content-Type: application/json
+    Authorization: Bearer eyJrIjoiT0tTcG1pUlY2RnVKZTFVaDFsNFZXdE9ZWmNrMkZYbk
+
+**Example Response**:
+
+    HTTP/1.1 200
+    Content-Type: application/json
+    {
+      "message": "Notification deleted"
+    }