SES-2337 updated refs to XSD 1.1 spec, tightened validation of legal dateTime values.

The dateTime validation code was out of date with RDF 1.1 specs (which refer to XSD 1.1, rather than XSD 1.0). This spec has much clearer rules about legal lexical space/value space for dateTime. Validation has been tightened to check for illegal month/day/day-of-month combinations (including checking for leap years).