Add simple demo application.

Add Demo application that can register to GCM, logs registrationId, gets and logs messages from provider.
Add opfpush-sample module.
Remove gcm_migrate_sample and simple_listener modules.
Add some logs.
Perform some refactoring.
Use package visibility for states checking methods.

Make visibility of states checking methods package.
Add isRegistrationAvailable() and isUnregistrationAvailable() methods for checking possibility of registration / unregistration.
Add getRegistrationId() method.
Add showing registrationId and list of receiving messages to sample application.