Add a colorblind friendly heatmap.

Here's a colorblind friendly heatmap from Wistia:
    http://wistia.com/blog/heatmaps-for-colorblindness

I made it available under the name 'wistia'.  You can see how it compares to similar colormaps here:
![download](https://f.cloud.github.com/assets/326005/2340227/1657d1e4-a4c0-11e3-8837-6945903cafe6.png)

Aside: It looks like many of the colormaps in `_cm.py` have "meta" information specifying what they were designed for. Perhaps this information should make its way into the colormap class, so that users could query the colormap for it?