onPreRenderGraph added to Rickshaw.Graph.Ajax

I have made a change to the Rickshaw.Graph.Ajax class to allow the implementor to modify the underlying Rickshaw.Graph before it is rendered. 

I added this because I was having issues trying to get the graph to display bars that were unstacked.  I now use this callback to set the appropriate property before it is rendered.  Setting after the render method was invoked had no impact.