Merge pull request #1 from Eumaeus/master

Minor edits.
Keep editing on replies.
Merge branch 'master' of https://github.com/neelsmith/cts-spec
Add placeholder for GetFirstUrn request.
Add text for GetFirstUrn request.
Update PrevNext schema.
Correct name of file.
Save final commit on prenext
Modify prevnext to allow for empty urn.
Work on GetLabel.
Update GetPassage* schemas.
Edits to GetValidReff, minor changes.