Nfl-data-py

Changed
- Added feature to cache play-by-play files locally
- Updated load_pbp_data() to work with locally cached files
- Updates README

Changed
- Removed local build files
- Removed soon to be deprecated setup.py `test` feature
- Synced tox and setup.py python versions
- Added new functions for pulling PFR, injury, snaps, NGS, and depth chart data
- Updated python requirements to >=py3.5
- Fixed bug in clean_nfl_data()
- Updated tests with new functions
- Update README to align with function updates