Handle importing the immediate setup.py.

[Robpol86]

Exception occurred: File "conf.py", line 6, in ImportError: No module named 'setup'

[Robpol86]

Maybe instead of handling importing the immediate setup.py, I should add a new API. Not everyone imports their setup.py into their conf.py, and when they do they might handle setting sys.path.append() in their own way.

To handle conf.py's with bad/missing sys.path.append() calls (e.g. v2.2.0 of this project :P) maybe let users define callback functions in their current conf.py (ignore conf.py's in git refs, just like scv_* variables are treated) that SCVersioning calls for each git ref after forking/spawning.