Better-looking histogram checkboxes

switched up the build process a bit more, so most of the build occurs in-place, meaning the src and dest options are the same. This allows us to simple copy everthing at once and not need to specify all the edge cases like I did previously in copy:everything_left_in_src

fix the path to the leaflet image directory

Make panel extra icons more obvious, fix overlap, add tooltips, add method for panels to define custom modals

Readded percent stacking, fixed bug when area = 0, added view options to histogram outside of editor

Cleaned up the Grunt config a bit, removed an unneeded dep

moved the aws config out of the general gruntfile setup, and into a task that is only run when needed.

Fixes for #466

Conflicts:
	panels/histogram/module.js

 - Created a grunt task to build, compress, and upload the build to s3
 - Updated the README to point users to downloads.elasticsearch.org