Fix #14, add setup.py and relative files

setup.py is added. To install dependencies, type

```
python setup.py develop
```

note that `develop` is different from `install`, as `develop` make a link to current package directory in .pth file and as `install` copy the whole package into site-package folder. We are developing, we want our change to code reflects without running install again, thus, we use `develop` rather than `install` here.

Also, it is better to create a virtual environment for running the project. You can type

```
pip install virtualenv
virtualenv --no-site-packages env
source env/bin/activate
python setup.py develop
```

In that way, the billy package and relative dependencies will be installed to the env/lib/ folder rather than the global one. This could help avoiding version conflicts and other issues.