Nlcodec

- Option to supply preconfigured `spark` session object
- Add docs

- Option to accept term frequencies as input
- PySpark backend to compute word and char frequencies
- `--min-co-ev` of BPE is CLI arg

- FIX: `find_packages()` in `setup.py` file to include nested packages

- uploaded to pypi : `pip install nlcodec`
- public repository with apache license 2.0