Improve social flow

Before the user had to manually sign in by tapping a non-obvious button
each time the app was opened. The user is now automatically prompted to
sign in when the app loads.

There is a status text in the bottom right that shows the current
social state. It bounces to show any changes.

In addition, the SocialManager class was added to abstract a lot of
the platform-specific logic away from the rest of the codebase.
Merge pull request #45 from pR0Ps/auto_signin

Automatically sign into GPGS