Giving Collections a clone() method.

backbone.js

@@ -791,6 +791,11 @@
       return resp;
     },
 
+    // Create a new collection with an identical list of models as this one.
+    clone: function() {
+      return new this.constructor(this.models);
+    },
+
     // Proxy to _'s chain. Can't be proxied the same way the rest of the
     // underscore methods are proxied because it relies on the underscore
     // constructor.