Initial commit of read_postgis function

Wraps read_sql() and looks for the specified geometry column which gets
converted to shapely geometries. It returns a GeoDataFrame.
Add testing for read_postgis function

It also installs postgis and creates a simple database filled with the borough
data. A new requirements.test.txt file is created so that not everybody
installing geopandas is required to install the packages for testing, psycopg2
in this case.
Skip database tests if psycopg2 or db missing

Check if psycopg2 is installed. In setUp() check to see if we can connect
to the test_geopandas database. Skip the read_postgis tests if either
fails.
Create the NYC borough database in setUp()

Also removes the nybb table creation in the travis script
Add documentation for read_postgis()
Open database connection within a context manager
Add documentation to explain database creation
Rename read_postgis() to from_postgis()
Merge pull request #25 from jwass/postgis

Initial commit of read_postgis function