Fixes my install problem.

To reproduce:

```
  git clone <this repo>

  virtualenv env
  source env/bin/activate
  python setup.py install
  cd ~/ (or anywhere the module won't be on the current path)

  python

  > import armstrong.esi
```

Will fail, as the `__init__.py` isn't installed.

I think you all know this namespace packaging stuff better than I do, so please let me know if there's a better solution here!