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)
This addition is interesting but I don't think it's essential. Would you mind submitting this addition as a separate PR and removing it from this one? I'm happy to include the earlier block since that is something that is kind of necessary.