Make pip installable

[cthoyt]

Closes #6

This PR does the following:

1. Adds a .gitignore
2. Reorganizes the code into the src/ layout (just renamed the codes/ directory)
3. Make imports absolute for project
4. Add setup metadata in setup.cfg and setup.py and update README with installation instructions
5. Use python entrypoints to make a command line interface and update examples/README accordingly

Minor tweaks:

-Add tqdm during training