* Prettify duration unit format
Format duration like "2 days, 19 hours, 48 minutes" and make Decimals useful e.g.
0 decimals: 2 days
2 decimals: 2 days, 19 hours, 48 minutes
3 decimals: 2 days, 19 hours, 48 minutes, 27 seconds
Previous code did some rounding that is now gone so numbers might be
different.
* Rewrite kbn.toDuration without moment.js
Because https://github.com/moment/moment/issues/3653
* Add tests for kbn.toDuration
