unified virtualenv for m-c

Currently, python lives all over the source tree and, as needed, is hacked together to see each other with http://mxr.mozilla.org/mozilla-central/source/config/pythonpath.py Instead, virtualenv (http://www.virtualenv.org/en/latest/index.html) should be used to keep m-c land python separate from system python and allow cross-module imports easily and in a more robust way than the current ad hoc solution. This should make it easier to add python code to m-c without modifying makefiles (much if at all) as well as tend to a more modular architecture and consuming third-party modules (where third-party could include mozilla authors).