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)

The code to produce this plot:
```

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.io import img_tiles

plt.figure(figsize=(14,12))

tiles = img_tiles.MBTiles('open-streetsz11.mbtiles')
# tiles = img_tiles.OSM()

ax = plt.subplot(311, projection=ccrs.Mercator(), title="Level 9")
ax.set_extent([-4.6, -2.6, 50.35, 50.65])
ax.add_image(tiles, 9)
ax.coastlines("10m")

ax = plt.subplot(312, projection=ccrs.Mercator(), title="Level 10")
ax.set_extent([-4.2, -3.0, 50.4, 50.6])
ax.add_image(tiles, 10)

ax = plt.subplot(313, projection=ccrs.Mercator(), title="Level 11")
ax.set_extent([-3.9, -3.3, 50.45, 50.55])
ax.add_image(tiles, 11)

plt.show()
```
You should use `io` here for future Py3k support.