Rewrite pre_workshop page to focus on the VM method, copy Dan's VM setup instructions to new page

Like the commit says, the instructions on how to init are mainly copied from the railsbridge/railsbridge-virtual-machine repo.

I'm using the rails 4.0 image S3 url -- don't know if we want to update the filename for the final version. I can also host a mirror if needed (i can set up a vhost for any subdomain like files.railsbridgeboston.org).

Stuff I'm wondering about:

0. Should we update the recommended VirtualBox and Vagrant versions to latest stable? The image works fine. Right now the Vagrant version recommended here is slightly ahead of the one in the railsbridge-virtual-machine repo.

1. can you even download Xcode 3.4 anymore? In any other situation, if someone walked in with a PPC or too-old-to-virtualize Intel Mac, I'd probably recommend installing Homebrew/Tigerbrew (OSX gcc command line tools only, not full Xcode) and updating Ruby that way, but I'm not sure it's likely any students will have a Mac that old. It's possible keeping that bit might just be distracting/confusing to people who don't need it.