Add a basic setup.py (fixes #4)

This allows newsbuilder and its tests to be installed.

To test, install newsbuilder in a clean virtual env. eg
```
mkvirtualenv newsbuilder-setup-4
pip install --editable .
```

And then run the tests and the build-news command.

```
trial newsbuilder
newsbuilder ~/projects/Twisted/trunk
```

Fixes: #4 
Perhaps worth considering eventually using setuptools entrypoints here.