Bug 1050478: Modify the flask-browserid package.

Modify the browserid package to use the "X-Forwarded-Proto" header
so that browserid audience matches. See:
https://github.com/garbados/flask-browserid/pull/21
Merge pull request #78 from mozilla/bug1050478_browserid_over_https

Bug 1050478: Modify the flask-browserid package.