Adding the ability to provide a transform function for transforming data

I've added the ability to provide a transform function that will be used on any data that is fetched.  This allows an existing REST service's data to work with this plugin without a change to the data returned by the REST service.

I've also made two small tweaks to the scope that is used when calling the 'url' function so that "this" is the plugin scope.  This is useful if one needs to get the current path (via this.miller('getPath')) in order to generate an appropriate url.  This seems to be congruous with the way toolbar.options and pane.options callbacks are executed.