Use middleman

This is where I plagiarise all of @puyo's work on the microcourses site and claim it as my own here.

Using middleman means we can have a single included header / footer, and use sass, and other cool stuff.

Don't merge it yet - there's still stuff broken, but I've got the home page working at http://joshuapaling.github.io/rails-girls-next/ (you can double check it's the middleman version cos I temporarily added 'made with middleman' in the bottom left)

Just submitting a PR now to see if @lbain want's to go with middleman or not? If so, I'll make more fixes and  we can merge when it's all converted.