Add [TBUserDefaults user], which returns the current user.

Persist the user in [TBUserDefaults userDefaultsForUnauthenticatedUser]
in a key called "USER".  This means that when the app starts again,
we'll know who was signed in last.

Add [TBUserDefaults setUserAndSynchronize].
Add [TBUserDefaults allRegisteredSettings].
Added [TBUserDefaults objectForKey:wasUnlocked:], and typesafe
equivalents.
Improve a comment.
Fix a critical bug in the situation where the plist doesn't
exist and we're creating a new one.
Added TBUSERDEFAULTS_REGISTER_OBJECT_WITH_DEFAULT,
TBUSERDEFAULTS_REGISTER_NSSTRING_WITH_DEFAULT.