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 99% 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.tests                                     0      0   100%
billy.tests.helper                             24      0   100%
billy.tests.test_models                         0      0   100%
billy.tests.test_models.test_company           87      0   100%
billy.tests.test_models.test_customer          96      0   100%
billy.tests.test_models.test_plan             114      0   100%
billy.tests.test_models.test_subscription     120      0   100%
billy.tests.test_models.test_transaction      100      0   100%
billy.utils                                     0      0   100%
billy.utils.generic                            23      0   100%
-------------------------------------------------------------------------
TOTAL                                         872     11    99%
----------------------------------------------------------------------
Ran 39 tests in 1.334s

OK
```

To install and run testing

```
virtualenv --no-site-packages env
python setup.py develop
python setup.py nosetests
```

Only merge this branch back when it is good to go.