Add venv module to create Python virtualenvs.

The pip and easy_install modules can already create virtualenvs. There are some problems with them:
 - they have diverged in possibilities (site packages)
- they only use `virtualenv` to create virtualenvs.

I want to use `virtualenv2` on some systems where I also have python 3 installed. This also supports the `pyvenv` built-in in Python 3.3.

(Note that I could also change the other two modules, but this is cleaner)