Microdata parser: updated the parser to the latest version of the microdata->rdf note (published in December 2014)

re-based microdata-to-rdf-second-edition from six_2to3 branch as described here: https://github.com/RDFLib/rdflib/commit/b082c4851840f4c2c83910f84331fb07fca16f4c#commitcomment-8975378

pull request to run the tests...