Make differentialPhotometry.py PEP8 compliant

Hey,
I found this repo through reddit's /r/astronomy, and I figured I'd try to contribute something.

First thing I'd like to discuss is if you are OK with me going trough the code and making it compliant with the PEP8 Style Guide [0], as it stands the code contains a fair amount of inconsistencies and can be a bit hard to read in general. While I understand and respect that you may have you own coding style, adopting the de facto standard that PEP8 is would make it easier for others to understand your code and contribute to it.

Matt

[0] http://www.python.org/dev/peps/pep-0008/