add zope.interface and Twisted, don't use the system ones

In OS-X, using --system-site-packages includes both
/Library/Python/2.7/site-packages (which, I think, is where "setup.py
install" goes, instead of /usr/local), and
/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python
, which holds python modules shipped with OS-X that aren't from python
itself. This includes Twisted, pyOpenSSL, numpy, and some OS-X specific
things like py2app, PyObjC, and bonjour.