Add cartopy and its dependencies

Tested on OS X and Ubuntu.

Includes fix for shapely using the wrong geos version on OS X:

	Assertion failed: (0 != cs), function GEOSCoordSeq_getSize_r, file geos_ts_c.cpp

https://groups.google.com/forum/#!topic/scitools-iris/FkJkxf1vcbA
https://github.com/cfarmer/python_geospatial/issues/3