Added more info in virtualenvs.rst.

Added the most important command line parameter, how to specify the Python interpreter to use. 

Also added some neat virtualenvwrapper tricks that are kind of easy to miss in [the docs](http://virtualenvwrapper.readthedocs.org/en/latest/command_ref.html)
@syndbg feel free to send a second PR. I'm not a fan, but Kenneth might like it.