IPython/Jupyter Rich Display Support

[corranwebster]

The IPython/Jupyter notebook display system looks for certain specially named methods to support their "rich display" system.

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.

[jwiggins]

Can we close this now that #389 has been merged?