Add python setup script

[Erotemic]

This adds the standard python setup.py file, which will allow users to install via pip install git+https://github.com/ucbdrive/bdd-data.git.

If you want help publishing this module to pypi, so users can just pip install bdd_data, let me know. My ubelt library: https://github.com/Erotemic/ubelt has a good template for doing this. See the publish.sh script.