Add support for OpenID extension.

This work originates from the work of @jeffmiller at https://github.com/mitsuhiko/flask-openid/pull/10
I added on the top of that the possibility to parse the OpenID response with the OpenID
extension of interest.

With this patch one can use flask-openid with an OpenID supporting the team or the CLA
extensions.
Sometime the OpenID server sends back information using POST queries.

If the number of information returned is too large, the server will send back information
using a POST query instead of GET. We should be able to handle both.

This situation can easily happens in large organization using OpenID for auth with the
openid-teams extension.