Reorganize code and make pip installable

[cthoyt]

This PR does a couple things, but mainly it closes #2. I did my best to leave helpful commit messages - I copied the most important ones here. I'd be happy to explain anything I did or give more insight on how you can continue to make your package more accessible for others!

Reorganize repository and clean up imports

• Switched to the /src/bionev structure because it's the community standard
• Added encoding line to beginning of each python file
• Made all imports relative to bionev
• Used PyCharm's "optimize imports" function to reorganize the order and remove unused imports

Make pip installable and relax requirements

• requirements.txt is only for non-packages, so now the requirements live inside setup.cfg
• The old requirements were pinned to very old versions of each library. I relaxed those requirements and let it install the newest versions of everything except tensorflow, and it seemed to work still.
• Updated the README with installation instructions

Add CLI entrypoint

• Used the entry_points to make it possible to run bionev from anywhere instead of needing python src/main.py
• Updated the running example in the README to use the new CLI

Adds More Evaluation Metrics

• I wanted to see the AUC-PR and MCC presented, so I added them to the output.
• I changed the eval_results.txt output to make the output in structured JSON. this will make programmatic analysis of results much easier in the future

[xiangyue9607]

Thanks so much! It's my first time to build such repo and I don't have much experience. I appreciate your help for making it better. Feel free to let me know if you have further suggestions.