Issue 624: support for drawing manager

added simple drawing manager directive, with support passing an options object directly to the google API, $watch()ing that options object to call the API on changes, disabling the $watch() for performance, and a control object to get direct access to the API DrawingManager instance.

Documentation has not yet been added to the website branch, but I'll work on that next.

@nmccready 
Nice is it possible to add bindings for tracking shapes/lines or just things drawn by the manager? This would be useful for lists and creation / deletion of shapes based of the state. I imagine some of this can be done entirely via script (via events for the drawing manager).  That brings up another topic should the events attribute be added for this as well?

Either way this is a great start.