Version 2.0.
Release 2.0

Updated for 2.0 version of Microdata to RDF Note.

* No top-level md:item listing top-level items.
* Limited registry semantics, now hard coded to `multipleValues=unordered` and `propertyURI=vocabulary`.
* Simplified vocabulary expansion, no longer results in `rdfa:usesVocabulary` triple.
* Integers and Doubles generated from `data` and `meter` element values.
* Support for experimental `@itemprop-reverse`