Io

This pull request addresses Issue #34.

* Moves functionality of GeoDataFrame's `from_file` and `from_postgis` into a new `io` sub-package.
* The functions are renamed to `read_file` and `read_postgis` to be consistent with `pandas` naming conventions.
* The functions are exposed at the top-level `geopandas` module.
* Introduces a `util` module in `tests` for performing some common operations across the tests, e.g., downloading the shapefile, connecting to the test database, etc.

I kept the name `read_file` since I'm not sure any of the options are better.