SES-1802 : Use language before datatype

To smooth the migration to RDF-1.1 where language literals will likely
have the datatype rdf:langString, this commit rearranges a large number
of locations that called getDatatype before getLanguage, and would
otherwise lose the language if they were executed as part of an RDF-1.1
path
Merged in SES-1804 (pull request #117)

SES-1801 SES-1804 : Cleanup unknown datatype handling and other non fatal errors
Merged in SES-1802 (pull request #118)

SES-1802 : Check for language before datatype to ease migration to RDF-1.1
SES-1807 : Add DBPediaDatatypeHandler
SES-1807 : Add constant for DBPedia datatype
SES-1807 : Fix NQuads test that used dbpedia datatype as invalid
SES-1807 : Add since annotation