ENH: Allow geometry property to be set.

* Allows for clearer errors when a GeoDataFrame is created but has no
  geometry.
* TST: Add test util for comparing sequence (b/c assert_almost_equal seems
  to not work with shapely.
TST: Move geom_equals and geom_almost_equals to tests/util and use them
Add assert_geoseries_equal test util
TST: Fix setattr issue with geometry property and more edge case tests
Merge pull request #54 from jtratner/change-geometry-v2

ENH: Allow geometry property to be set.