Remove sys.path manipulation

This way the layout is more like a standard Django project layout. Previously Django added the project's path to sys.path, meaning you could refer to `metric` or `graphite.metric`.

I believe `manage.py` was the only place something was referenced without the proper `graphite.` prefix.

In development, one can run graphite-web by cd-ing to the `webapp` directory and run `./manage.py runserver`.