Properly parse .pth files. Modify pythonpath before tests are imported

Tried to use this package and it wasn't working, found two issues:
1) Directly modifying sys.path doesn't traverse .pth files (see https://docs.python.org/2/library/site.html)
2) pytest_configure doesn't get called early enough for me, had to use pytest_load_initial_conftests.

I don't expect you to accept this pull request due to my second modification (pytest_load_initial_conftests). It's a pretty questionable change, my need for it is the result of a settings.py file that depends on the python path being updated before it's imported.