Fixed broken link from README to contributions page.
Remove Persona auth from simple login

Mozilla is no longer actively developing Persona so we are removing it
from all of the simple login clients.
Merged with master
Prepping for version bump to 2.0.0
Changed version number to 1.5.0
Removed out of date tests which somehow escaped the merge with master
Merge pull request #28 from firebase/remove_persona_auth

Remove persona auth