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.
Fixed broken link to contribution guidelines in README
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Removed out of date tests which somehow escaped the merge with master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
Fixed incorrect version number in change log
Merge pull request #30 from firebase/update_changelog

Fixed incorrect version number in change log
Merge branch 'master' into popup-option-window-keepalive
Update version, changelog