check language before datatype to work with rdf-1.1

add more tests to verify behaviour for datatype and language literals

some of the tests should start to fail from sesame-2.8 as plain literals become xsd:string
Complete testCompareLiteralObjectsTyped
add static instances for StatementComparator and ValueComparator

also use Literal.getLabel instead of stringValue
work on testCompareLiteralObjectsTypedAndLanguage
complete testCompareLiteralObjectsTypedAndLanguage
Merge pull request #36 from ansell/feature/rdf-1.1-comparators

Make comparators compatible with RDF-1.1