wip

finish adding Id()
Introduce tag.Id() and tag.Kind() methods.

Prepare to alter the signature for ParseTag to return (Tag, error)
Introduce tag.Id() and tag.Kind() methods.

Prepare to alter the signature for ParseTag to return (Tag, error)
Merge branch '104-introduce-tags-type-ii' of https://github.com/davecheney/names into 104-introduce-tags-type-ii
responding to review feedback
Merge pull request #3 from davecheney/104-introduce-tags-type-ii

juju/names refactor part 2