Make it possible to programmatically add annotations to "urn:nuxeo:reponame:docref" directly

Right now the AnnotationServiceProxy is trying to parse the "annotates" field as a URL before converting it to the internal "urn:nuxeo:reponame:docref" format. While this is fine from a GWT client it's akward to pass a public URL from a core event listener (that is not aware of any web context) for instance.

I propose to make it possible to bypass the URL check if the annotations URI already has form "urn:nuxeo:reponame:docref".