Rewrite, PyPI package, tests, git2json, docs, LICENSE

I totally dig the idea of this program but rewrote it to be easier to install and work with (at least for me).

Summary:

* It is now ready to be distributed via PyPI/pip (so installation would be `pip install gitcoach`)
* It uses [git2json](http://github.com/tarmstrong/git2json) (which I wrote) for parsing git logs
* It has nice argument handling
* It passes `flake8`
* It has documentation: installation instructions, example output, contributors' guide.
* It has some tests
* It includes the fix from #4

I also added a `LICENSE` file (BSD-3) with your name in it for convenience. I realize this is presumptuous and against the rules. Don't click the "merge" button if you don't like that or don't like that license. ;)

Also, I volunteer to maintain this program because I think it is cool.