Remove the need for PYTHONPATH mucking

I'd hoped that I could just switch to relative imports, but these don't work from scripts which you invoke directly (they only work from modules). There's been lots of discussion of this online and I think this is the simplest solution.
http://stackoverflow.com/questions/4348452/python-packaging-for-relative-imports/10713254#10713254
http://stackoverflow.com/questions/72852/how-to-do-relative-imports-in-python
http://legacy.python.org/dev/peps/pep-0366/

The win of making this change is that we can have files named `github.py` in other projects :)
and here