matplotlib issue in osx

[briantjacobs]

When trying to use the .mpl_colormap functionality even with matplotlib installed, I was getting

File "/Users/brianjac/.virtualenvs/bird/lib/python2.7/site-packages/palettable/palette.py", line 99, in get_mpl_colormap
    raise RuntimeError('matplotlib not available.')

I'm using a python virtual environment. Turns out I needed to do this step:
https://stackoverflow.com/a/35107136/4863986

This took some sleuthing, maybe worth mentioning in the documentation, as matplotlib is needed to use certain functions like .mpl_colormap

I would also love to see some more cookbook examples on how to use this library for generic lists of interpolated colors not feeding into matlab plots.

Otherwise, great library, thanks!