mbtiles

This PR contains code I received from @iedwards for plotting map backgrounds from an mbtiles database. The source data is a [6GB mbtiles database] (https://drive.google.com/file/d/0B1nV2W9moCwwaFpsZzVxZ1lqOGs/edit?usp=sharing) created by a colleague, so I'll just include an image of the output:

![mblatlon](https://f.cloud.github.com/assets/2048530/2049287/7c7b488e-8a5d-11e3-8a47-5081f2ee6e7f.png)