External transform api - #1090

This PR adds an easy interface for matplotlib transform creation based on a custom object.

The syntax looks like:

class MyObject(object):
    def _as_mpl_transform(self, axes):
        return ... # a matplotlib transform instance

This interface is comprable to the _as_mpl_axes method to easily create axes from custom classes.

My primary usecase for both of these cases is to provide an interface which could do something like:

plate_carree = MyProjectionLibrary.PlateCarree()
robinson = MyProjectionLibrary.Robinson()

ax = plt.axes(projection=robinson)
plt.plot(lons, lats, transform=plate_carree)