Suggest virtualenvwrapper instead of vanilla virtualenv.

I suggest these changes mostly because I dislike the "virtualenv geordi" instruction in the README.md.  geordi is already the project folder, making it also the virtualenv folder makes it very difficult to switch to a different virtualenv or just remove the virtualenv.  The virtual environment should not be in the source folder at all, or if it is, it should be in a clearly marked directory which can be easily "rm -rf"-ed and easily added to .gitignore.

I suggest virtualenvwrapper because you don't have to worry about where the virtualenvs are, it takes care of that.  You just give it a name.