Add prototype DrawBox.

Pretty simple right now -- all we're doing is drawing circles and
saving them. But it works! The interface can definitely be
extended/improved -- colors, more control over shapes, etc. Also, it's
worth questioning whether or not we want to allow the notebook owner
to draw an initial image for the user to start from.

Side note: code duplication between backbone models is pretty bad
right now. Might want to do some refactoring soon to handle that.