Changes for schema.org 1.6

Changes for v1.6, as summarised in test build circulated 9th June - http://lists.w3.org/Archives/Public/public-vocabs/2014Jun/0106.html - in particular, adding Role type and supporting vocabulary, alongside site navigation support and better documentation for inverseOf and sub-property/super-property links.

In addition, improved testing to help avoid duplicate or missing term definitions, and merge of duplication for several properties: member, memberOf, acceptsReservations. Other 1.6 fixes include startTime/endTime duplication; replacing departTime mis-defined property.

The property valueName is now on PropertyValueSpecification and not EntryPoint.

Added isEnumerationValue() to API, with supporting tests.