Add support for Portier.io

[Natim]

This is exactly the kind of piece of software we are looking for Kinto Authentication: https://github.com/portier/portier.github.io/blob/master/Using.md

[leplatrem]

Are you sure it's an easy-pick? I don't see it as such...

[Natim]

Yes it is basically FxA authentication policy but for portier :)

[callahad]

Just for the sake of cross-referencing, it looks like work on this is happening in Kinto/kinto-portier#1