Move the virtualenv install stage to the docs

Fixes #87 

This has diverged from @itamarst 's suggestion to make a pre-setup script (which would likely install `virtualenv` differently on OS X and Linux) and then have a shared installation script. It instead keeps the separate script and asks the user to install Homebrew and virtualenv manually.

This is, of course, not as simple as just running a script, but in testing it proved (to me at least) to be better - installing Homebrew requires you to hit the return key, then accept a dialog, then enter your password.

Also, installing a package manager in our own install script isn't very friendly I think. Although we do advise Homebrew later, in the tutorial, it is not a requirement. Firstly, you could be installing flocker-cli and not do the tutorial, and secondly you can install `mongo` without Homebrew.
My comment above would become incorrect if https://github.com/ClusterHQ/flocker/pull/486/files#r15987777 is "actioned".