Made django-comps work with python3.3 and Django > 1.4 < 1.7.

Also changed tox to test these environments, but not test against trunk. There was a seemingly unrelated failure in Django 1.7 and for the purpose of this pull request, only Python3 and Django <= 1.6 support is needed. I suspect the  Django 1.7 work won't be *too* extensive

I removed the commented out media tree building part of the zip export view, and removed the /media path rewriting there as well. I think some more simplification here might be possible but is better left to future work.