Changed #plain? to allow for language-tagged literals, which may also have rdf:langString datatype. Also non-language-tagged literals may have xsd:string datatype.
Changed #simple? to just be no language or datatype.
Added Literal::Numeric#abs, #ceil, #floor, and #round; used to be just on Double. #floor is new.
Add #tz and #timezone to Literal::DateTime to support SPARQL 1.1 functions.
As per SPARQL 1.1, querying with no patterns returns a single empty solution.
Use RDF::Spec.triples to get access to test data rather than reading it from a local file.