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)
Alright, removed the non-essential info.

Should I still submit a second PR for virtualenvwrapper command? If you think it's unnecessary, we can leave it out. People can still find it in the docs.