Make basemap work with mpl 1.4 egg directory as generated by setuptools

Starting in version 1.4, mpl is installing mpl_toolkits inside
its egg. To work with that, basemap needs to declare mpl_toolkits
as a namespace package, and put a suitable __init__.py in it.
Merge pull request #116 from efiring/namespace

Make basemap work with mpl 1.4 egg directory as generated by setuptools