add package_dir argument to setup()

`import ordrin` fails when this package is installed via `pip install ordrin`, because `schemas.json` is not included in the distribution. That is happening due to the fact that `setup.py` is only looking for `.json` files in the distribution root, rather than in `/ordrin`.
This tells `setup.py` to look for `.json` files in `/ordrin`.

For more info, see: http://docs.python.org/2/distutils/setupscript.html#installing-package-data; and http://docs.python.org/2/distutils/sourcedist.html#principle.