Non-rdf resources emit /fcr:metadata events instead of /jcr:content

Fixes https://www.pivotaltracker.com/story/show/81279166
+1 : use constants (FedoraJcrTypes.FCR_METADATA, plus maybe an addition for JCR_CONTENT in RdfLexicon).

My expectation would be that: 
- updates to binaries would send events with the binary path, and
- updates to binary descriptions would send events with the path to /fcr:metadata.

Is that a shared expectation?