FEATURE: non-supported browser "then" support

[stevebuik]

Lightning provides promises for supported browsers but what about un-supported. For Lightning running in sites/communities, users could use any browser.

Ideally tests with promises can run for any browser so that they can be used to check all browsers (which can be automated using Sauce Labs)

It should be possible to check for Promise and, if not there, then include an extra lib that implements it for all browsers (looking at you IE8)

The tricky part will be finding a lib that works in LockerService. I already tried bluebird.js but couldn't make it work. The alternative will be to write a simple implementation with just enough then support - there are articles on the web showing how to to do this.