Add Twitter API integration to demo app

[sharplet]

There's a bunch of stuff here, but here's a quick summary:

• Refactors the demo app to support showing either GitHub or Twitter profiles
• Builds up Twitter authentication and authorization step by step
• Includes a tiny Rack app that serves as an OAuth redirect to the superbdemo: URL scheme, and is deployed to Heroku
• Minor public API change (non source breaking) that allows AuthenticationProvider.authorize() to throw
• Some other minor changes.

Check out the commit log for more info!

[sharplet]

@gfontenot Yeah that's the idea, I need to complete #14 and also extract this into a new framework.