BF: nxutils.pnpoly() can have problems, so add try-except

matplotlib 1.2.0-r1 on gentoo does have nxutils, but raised:
AttributeError: 'numpy.float64' object has no attribute 'frozen'

fix: try-except will now fall through to pure python.
long-term, might be better to figure out what is going on, might be a more
general issue for matplotlib 1.2.0-r1 (I use matplotlib 1.1.0)
Merge pull request #244 from jeremygray/master

nxutils can be present but not work for point-in-polygon stuff