IPython/Jupyter Rich Display Support

The IPython/Jupyter notebook display system looks for certain specially named methods to support their ["rich display" system](https://ipython.readthedocs.io/en/stable/config/integrating.html#rich-display).

For Kiva graphic contexts which have output compatible with the display types that are supported, it would make sense to add these methods. This would be, in particular:

- image-based backends which can produce PNG formatted output should support `_repr_png_()`
- the SVG backend should support `_repr_svg_()`
- it may make sense for the PS and PDF backends to support `_repr_mimebundle_()` and return `application/postscript` or `application/pdf` as appropriate, although it is not clear if Jupyter will do anything sensible with them

The first two are implemented in #389, but it needs tests.