[DOC beta] Cleanup RSVP documentation.

Sync RSVP docs from https://github.com/tildeio/rsvp.js/pull/195.

Some screenshots of the integrated docs:

Module listing:

![screenshot](http://take.ms/iTl8m)

`Ember.RSVP` Index:

![screenshot](http://take.ms/p4o8V)

`Ember.RSVP.Promise` Index:

![screenshot](http://take.ms/SzyAC)

`Ember.RSVP.EventTarget` Index:

![screenshot](http://take.ms/xdVgG)