Adding back Backbone.$

http://backbonejs.org/#Utility-Backbone-$

Also removed `setDomLibrary`

Backbone change log 0.9.9 — Dec. 13, 2012:
To set what library Backbone uses for DOM manipulation and Ajax calls,
use `Backbone.$ = ...` instead of `setDomLibrary`.
Overloads for Collection.get method
How to access attributes in a strongly-typed manner

Only added comments and examples
Updated collection tests and added comments
Added test for adding object literals as models
Merge pull request #2523 from omidkrad/master

Backbone updates and comments
Merge remote-tracking branch 'upstream/master'