Rewrite for Rails 3.1+ and Ruby 1.9+

Hello @ugorelik (or whoever at @HealthyWeb is maintaining this currently),

First of all, thank you very much for this gem! I sat down this evening looking for a markItUp gem, and you showed up with the canonical name. I had some updates in mind to match some newer project conventions ... and I ended up completely rewriting it! :smiling_imp: :sunny: :confetti_ball: :haircut: 

You can see all the neat new :sparkles: things in [the README](https://github.com/phlipper/markitup-rails/tree/rewrite#markitup-rails), and the commit messages should be fairly informative, but I did break Ruby 1.8.x and Rails 3.0.x support. I don't know if this is critical for you, and if so I can just release this as a new gem with another name, but I thought I'd take a chance and see if you were interested.

Thanks for your time and consideration!