Initial commit of the new user experience:

 * Removed old account setup and signup activities
 * Added WelcomeActivity: This activity is displayed at app startup and whenever the user signs out/in again.
 * Added TutorialActivity: This is a one time use activity that walks the user through features of the app.
 * Added NewAccountActivity: This activity creates a new WordPress.com account over the REST API. Props @daniloercoli

I still have some TODOs but this is the start :)