Allow backdrop to be packaged with setuptools

This branch is not for merging it's more of a conversation starter.

This allows us to build backdrop as versioned source distribution that can be installed and managed with pip.

To build a source package.
```shell
python setup.py sdist
```

You should now have a directory called `dist` with a tarball called something like `backdrop-0.1.0.tar.gz`

What are peoples thoughts?