Support for differently-named settings files or structures

I use `project/settings/dev.py`/`project/settings/production.py` or something similar in a lot of projects. 

This PR provides a way to explicitly set the project root path, instead of only the magic `replace('.settings', '')` method.

Thanks for a great project!

If this looks good and gets merged, a quick version bump and pypi release would be awesome, so I can pull our projects off my fork and back on to trunk.  

Thanks!