Adding Curate.permanent_url_for(object)

By providing a Curate.permanent_url_for method I am hoping to convey
that Curate's /show/:noid URL can be viewed as permanent and thus
we can point DOIs at that URI.

Also updating the generator to include a clue about the permanent URL.
The permanent URL is now required.

Closes ndlib/planning#204
Closes ndlib/planning#205
Merge pull request #190 from ndlib/planning-204-doi-purl

Adding Curate.permanent_url_for(object)