Use custom inline code for virtualenvs
This removes the reliance on the virtualenv package's activate_this.py so that other virtualenv tools can be used Based on the code from activate_this.py, this includes only the parts which don't already get inherited as environment variables
See #402 (closed) for the problem, which I think this fixes, but I could be missing part of what's necessary to set up virtualenvs.
Edited by Benjamin Winger