Ember.reduceComputed tolerates initially null values.

Fix #3255.
Ember.reduceComputed remove in reverse order.

This makes simple implementations of things like `filter` easier to reason
about.

Fix #3272.
Merge pull request #3284 from hjdivad/array-computed-bug-fixes

Fix a couple of arrayComputed bugs.
Implemented Ember.ajax

and Ember.sendJSON

Identation

Removed evented stuff

Always use RSVP.Promise