Rewrite

Restart from a solid ground up

Basic models are built (core business logic and RESTful API are not implemented yet), still not in functional stage. However, we have 97% testing coverage currently. I hope I can keep this through and eventually deliver with 100% testing coverage.

```
Name                        Stmts   Miss  Cover   Missing
---------------------------------------------------------
billy                           0      0   100%
billy.models                   19      9    53%   19-36
billy.models.company           37      0   100%
billy.models.customer          36      0   100%
billy.models.plan              48      0   100%
billy.models.subscription      45      1    98%   89
billy.models.tables            81      1    99%   35
billy.models.transaction       42      0   100%
billy.utils                     0      0   100%
billy.utils.generic            23      0   100%
---------------------------------------------------------
TOTAL                         331     11    97%
----------------------------------------------------------------------
Ran 39 tests in 1.282s

OK
```

To install and run testing

```
virtualenv --no-site-packages env
source env/bin/activate
python setup.py develop
python setup.py nosetests
```

Only merge this branch back when it is good to go.