Many minor improvements

Thanks for sf2github.

As I was using it, I ran into a number of small issues and also noticed a few improvements that could be made as I was troubleshooting. I created a series of changesets, many of which could be cherry-picked, or which may be accepted wholesale.

Highlights:
JSON format (Sourceforge default) now described as primary mechanism.
Python 2.7 or Python 2.6 + argparse now required.
Fixes when certain fields aren't present.
Load user-map from JSON file.

As it turns out, I'm probably not going to use this script for migrating my project, as the documented limitations mean the result isn't clean enough for my tastes. Nevertheless, I wanted to contribute this back to perhaps help others.