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 
Ok. Confirmed.
```
(newsbuilder-setup-4)[~]$ python ~/projects/Twisted/newsbuilder/setup.py  install
Traceback (most recent call last):
  File "/home/richard/projects/Twisted/newsbuilder/setup.py", line 44, in <module>
    long_description=read('README.rst'),
  File "/home/richard/projects/Twisted/newsbuilder/setup.py", line 14, in read
    with open(path) as f:
IOError: [Errno 2] No such file or directory: 'README.rst'
```

Pip doesn't seem to have this problem though. I guess it changes CWD or something.

I'll make the suggested change.