* 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.
Adds tests for the read_postgis methods. A tests/util.py is introduced
containing some of the common code for downloading the NYC boroughs file,
managing the existence of psycopg2, and connecting to a database.