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`.

backbone/backbone.d.ts

@@ -359,7 +359,7 @@ declare module Backbone {
 
     // Utility
     function noConflict(): typeof Backbone;
-    function setDomLibrary(jQueryNew: any): any;
+    var $: JQueryStatic;
 }
 
 declare module "backbone" {