do not rely on return exiting the function

- return does not actually escape the function on OS x
add . to PYTHONPATH
Merge pull request #71 from NarrativeScience/feature/bash-the-posix-snake

In which we bash the snake on OS X and add . to PYTHONPATH when snakeskinning
Headless repos now handled properly.

The test for headless now works, plus, we don't call get_branch()
twice for no reason.