Rails 4 Support

Updates codebase to support Rails 4, which was released in its stable form yesterday.

BC, this might be a tricky one to push to Heroku, but if you can do it, it'd be great. As best as I can figure, the easiest way to push this would be to, once the code is on Heroku, run `bundle install` and `bundle update` to force Rails to update to 4.0, along with its various dependencies. Unless either of those throw any errors, this should make the code good to go. If you can't do it on Heroku for whatever reason, the code for Rails 3.2.13 still works fine.

Strictly speaking, this upgrade doesn't really buy us much, but I just found it a fun intellectual exercise to upgrade to the latest version of Rails.