Added 'interpolation_none_vs_nearest' example

@tcaswell suggested (https://github.com/matplotlib/matplotlib/issues/2972) I put together this example when I realized the difference between interpolation = 'none' and interpolation = 'nearest' in imshow().  The docstring for imshow() mentions this difference, but pictures are worth a thousand words.

![nearest_vs_none](https://cloud.githubusercontent.com/assets/4099759/2743277/5acc9d28-c711-11e3-9a0f-8071df75aeec.png)

![nearest_vs_none_pdf](https://cloud.githubusercontent.com/assets/4099759/2743279/62ba2884-c711-11e3-8b56-916982b23768.png)