Merge pull request #2759 from machty/promise-startup-fix

Fixed bug with promises on startup. Fixes #2756.
Remove ember-testing from production build
Merge pull request #2761 from teddyzeenny/ember-testing-build

Remove ember-testing from production build
adding docs for Ember.beforeObserver
Merge pull request #2765 from twokul/master

adding docs for Ember.beforeObserver
Async transitions

This is the Ember side of

https://github.com/tildeio/router.js/pull/19

Not yet ready to pull, just giving folks an opportunity to play around
with the new async router API.