Ginga gets a Matplotlib backend!

Ginga can now render to a Matplotlib FigureCanvas.  The performance using
this backend is not as fast as the others, but it is acceptable and opens
up huge opportunities for overplotting.

See scripts/example1_mpl.py
Adding drawing canvas capabilities to Matplotlib-based Ginga

Partial commit--work in progress
Completed FitsImageCanvas for Matplotlib backend

- Usual Ginga drawing can now be done when Matplotlib backend is chosen
  - this uses mpl Artist classes
  - updated examples--see scripts/example2_mpl.py
- Can now draw compasses
- updated release number