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
changed #col= to _col=

see discussion in ISSUE 33: https://github.com/w3c/csvw/issues/33
revision of RDF mapping document

ready for review and comment …
Merge pull request #108 from 6a6d74/rdf-edit

Updates to the RDF mapping doc are now complete