Updated the installation instructions for Windows

Hey Lynn

The recent release of virtualenv 1.10 invalidated the installation instructions for Windows. I updated them, simplified them, and added virtualenvwrapper-win so that windows users can also have the `workon` goodness.

While I was at it, I updated the version of the linked python installer.

Installing pip and virtualenv on Windows is a pain. There's a PEP to include a pip bootstrap in python for 3.4, so hopefully, the problem will be solved someday.

I didn't update the html files for fear of doing it wrong. You'll have to do it after merging.

Thanks a lot for the tutorial, by the way !