ENH: Allow geometry property to be set.

Setting a property adds some tricky issues with DataFrame, this fixes it
by catching the KeyError as AttributeError. I still think
it has some useful possibilities (especially in terms of allowing
different column names to draw on for 'geometry'), but I haven't added
any of that yet.

* 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.