pidgin/pidgin

Add PurpleCredentialProvider as the first piece to replace the existing keyring api.

Testing Done:
Added some new unit tests and made sure they passs.

Reviewed at https://reviews.imfreedom.org/r/144/
REVIEWBOARD_URL = "https://reviews.imfreedom.org/"
REPOSITORY = "pidgin/pidgin"
REPOSITORY_TYPE = "mercurial"
LAND_DEST_BRANCH = "default"
LAND_PUSH = False