Added a login view controller.
Added an API client and API secrets files.
Added a singleton mechanism based on current Swift limitations.
Setup root view to switch Login/TweetList view based on Authorized state.
Twitter OAuth 1.1 authorization wired up.

1. Added OAuth callback handler and custom URL scheme.
2. Added authorize() function to TwitterClient.
3. Wired up LoginView.onConnect to TwitterClient.authorize().