Updated .gitignore file.
Added GTWExpression implementation of numeric ops, ABS, STR, REGEX, LANG, LANGMATCHES, and ISNUMERIC.
Added detail to debugging messages in GTWQueryPlanner error code paths.
Removed GTWSPARQLResultsSerializer protocol (replaced with use of GTWSWBase protocols).
Updated SPARQL Results serializers to use implement GTWSPARQLResultsSerializer protocol.
Updated GTWSimpleQueryEngine to modularize code and support more query plan operators.
Updated SPARQL test harnness code.
Added SPARQL Results XML Format parser implementation.