Canonical SSO provider

Comment 3 for bug 589351

This will be done by adding new, optional type argument to api.captcha.new():

>>> captcha = api.captcha.new(type="image/jpeg")

or

>>> captcha = api.captcha.new(type="audio/mpeg")

Currently that will be only two supported types, and type argument will default to "image/jpeg". Also this change requires additional argument in api.registrations.register() call, captcha_type defaulting to the same as type ("image/jpeg").