1) Deal in fully qualified URL resources through the rest
interface. All rest methods are now passed the complete url in
request->url.
2) Create rest::resolve() which lets individual resource definition
code convert a full url into the appropriate matching resource.
Implement gallery_rest::resolve() and tag_rest::resolve()
3) Reimplement tag_rest's get() and post() methods. They're much
simpler now.
4) Implement the tags_rest helper which supports working with the
entire tags collection.