Improvements to mouse navigation

1. Change the icon over the rendered image to indicate that it is interactive.  It might be nice to have different icons for rotation/panning for when the shift key is pressed if someone can come up with some good icon images.

2. For mouse dragging events, bind to document rather than the canvas.  This allows the drag events to continue firing even when the mouse leaves the canvas (or even the browser window).  I did this the only way I knew how, you'll have to tell me if it isn't sufficiently "backbone-esque".