Upgrade to Capybara 2

Hi,

Those cukes failing with Capybara 2 were bothering me. The main issue was that Capybara 2 doesn't look for text in hidden DOM elements anymore (which makes sense!) so we now need to use a JS-enabled driver. I went ahead and picked [capybara-webkit](https://github.com/thoughtbot/capybara-webkit). [Poltergeist](https://github.com/jonleighton/poltergeist) is cool too because truly headless thanks to PhantomJS but it requires Ruby 1.9.2+. You might want to use [another one](https://github.com/jnicklas/capybara#drivers). I also tweaked a bit the fake Rack app to serve other resources such as JS files. 

There's probably a minor overhead (although I can't see any difference locally) running cukes with JS but I thought it'd be more relevant for integration tests.

Also references 239db29e.