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
changing the way 'bounds' is implemented to match shapely
Merge pull request #35 from jwass/io

Io
Initial commit of geocoding functionality

Geocode a Series or list of strings and get back a GeoDataFrame with the
Point objects and a full geocoded address.
return to original bounds name and create total_bounds property
BUG: Use axes if passed in to plot series, and pass params from plot_dataframe
fix typo
ENH: Use existing figure if there is one
ENH: Use colormap for plotting lines
ENH: Pass column colors for linestrings
ENH: Handle MultiLineString geometries when plotting
Change assertIn to assert_ for python 2.6 support
Catch geocoding erros and fill with empty rows

Catch GeocoderResultError and ValueError. Fill the resulting GeoDataFrame
with an empty row.
Add some more documentation about geopy
BUG: pass alpha paramete through plot calls
BUG: Return axes from plot_dataframe and explicitly draw figure for both plot functions
DOC: Finally some docstrings for the plotting functions
ENH: Allow parameter for setting alpha for series plot