Added Persona auth deprecation warning

Mozilla is no longer actively developing their Persona authentication.
As a result, we are deprecating it and it will be officially remove at
the end of May, 2014 with version 2.0.0. This change adds a warning
every time Persona auth is used telling people about its imminent
removal.
Merge pull request #24 from firebase/persona_auth_deprecation_warning

Added Persona auth deprecation warning