Fixes #1468 -- add a 'request' event that allows folks to watch for s…

…pinners etc. Passes the XHR on which you can .then, .error, and so on.

backbone.js

@@ -1475,7 +1475,9 @@
     };
 
     // Make the request, allowing the user to override any Ajax options.
-    return Backbone.ajax(_.extend(params, options));
+    var xhr = Backbone.ajax(_.extend(params, options));
+    model.trigger('request', model, xhr, options);
+    return xhr;
   };
 
   // Set the default implementation of `Backbone.ajax` to proxy through to `$`.