Persist OriginalRecord

This is a substantial rewrite of OriginalRecord to allow them to persist to LDP.
Possibly both, but it may be mostly academic.

When you `PUT` an LDP-NR (i.e. file), an LDP-RS is created at the request URI, and the LDP-NR is created at whatever file extension Marmotta decides is best for the content type.  

This code manages the change in URI to the generated LDP-NR. If you create a new `OriginalRecord` with the URI where the LDP-RS got created, this method will get called, and `#rdf_subject` will get set to the associated LDP-NR.  

Fine so far, and it's reasonable to assume that we will want to maintain a one-to-one relationship between the generated LDP-RS/Containers and their contained files. Multiple files could probably be handled by `PUT`ing a new -NR at a new URI.

I think, however, if the `OriginalRecord` changes content type, and you try to put it to the -NR URI (which is what this code will try to do), you're going to hit 4xx conflicts.  That may be the thing we need to resolve eventually.