Issue #278. Allow 'extend' to be overrideable+inheritable.

backbone.js

@@ -931,7 +931,7 @@
   // The self-propagating extend function that Backbone classes use.
   var extend = function (protoProps, classProps) {
     var child = inherits(this, protoProps, classProps);
-    child.extend = extend;
+    child.extend = this.extend;
     return child;
   };