Add gems rdf-rdfa and equivalent-xml
Set context source to schema.org and creator to peter_v

* inspired by code from ruby-rdf project
Reading the Schema.org predicates (uri, label, comment)

* Happily reusing some code from ruby-rdf/rdf
  (was unlicensed under public domain)
* this is WIP, the extract need to be a static list
  (but this dynamic implementation lays the basis for the tests)