Escape dollar-signs in password for sync config file.

The sync config loader treads $X as an environment variable
substituation so we have to escape it when used in a password.
Use gunicorn for serving the syncserver app.

Gunicorn has better support for X-Forwarded-Proto than pserve.
Merge pull request #79 from mozilla/rfk/syncserver-tweaks

Some tweaks to syncserver setup