Add a basic setup.py

This allows newsbuilder and its tests to be installed.

It depends on #2 and #7 so please review those first.

To test, you can merge those into this branch temporarily using the following commands:

```
git merge --squash cleanup-1
git stash
git merge --squash readme-3
git stash pop
```

After which you should be able to 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
./bin/admin/build-news ~/projects/Twisted/trunk
```