* 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