Move core IO functions top-level geopandas

Moves the core functionality of GeoDataFrame.from_file and
GeoDataFrame.from_postgis to a separate io subpackage. The corresponding
functions in io are called read_file and read_postgis.
Add tests for io functions.

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.
Rename ogr.py to file.py. Remove unused import
Merge pull request #35 from jwass/io

Io