[OpenID] Signing method for XRD

[Hans Granqvist]

It seems the major hurdles to using xmldsig are "it's too hard to
implement" and "there is no good API in the {foo, bar, baz, ...}
language"

Since we envision a world of interconnected services, is it not easy
to avoid all issues by creating and set up a signing/verification
service endpoint. Internal or external, just use this service for all
nasty xml signature needs. I've done this myself a few times and use
it wherever http is spoken. It's pretty easy.

Hans