ENH: Add geometry property to GeoDataFrame

Adds a geometry property and set_geometry() method to GeoDataFrame. This was
suggested in Issue #45. The implementation mimics that of DataFrame's
set_index in that it returns a copy of the GeoDataFrame by default with the
new geometry. You can specify a column or give a list/ndarray of geometries.
Merge pull request #47 from jwass/set_geometry

ENH: Add geometry property to GeoDataFrame