predicateUri and default properties

see #101
call it predicateUrl
enable use of `predicateUrl` for RDF properties

fixes #95
clarified base URL for metadata

see #91
Merge pull request #108 from 6a6d74/rdf-edit

Updates to the RDF mapping doc are now complete
a few minor changes to the JSON mapping doc
Merge pull request #110 from 6a6d74/json-edit

a few minor changes to the JSON mapping doc
Merge pull request #111 from 6a6d74/rdf-edit

closed issue 100 & removed from doc