New formula: basemap

The matplotlib [basemap] toolkit is a library for plotting 2D data on maps in Python.

I propose to include it as a formula in homebrew/python since:

- it requires the following homebrew packages: `matplotlib`, `geos`, `pillow`, `numpy`
- it is cumbersome to install with `pip`: `pip install --allow-external basemap --allow-unverified basemap basemap`

I made this formula taking inspiration from `matplotlib.rb`. Please let me know if there is something missing or not correct.

[basemap]: http://matplotlib.org/basemap
presumably the other Python deps (matplotlib, pillow) need this treatment as well