Make the repo git-submodule friendly

Added an empty `__init__.py` so that the repo can be imported as a git-submodule in another projet like so:

```bash
git submodule add git://github.com/sixohsix/twitter.git twitter
```
so we can use: 
```python
import twitter
```
in the project where it was imported without the need to install it system-wide on the machine (when it's used for developpement purpose only)