Updated GTWTree class to implement the GTWRewriteable protocol.
Implemented support for planning and evaluation of EXISTS expressions.
Refactored numeric operator code in GTWExpressionEvaluationContext.
Implemented support for ENCODE_FOR_URI.
Fixed evaluation of ORDER BY clauses to accept expressions, not just variables.
Updated plan trees to use the .treeValue tree property.
Fixed planning of triples with blank nodes (now rewritten as plan variables).
Fixed handling of aggregate expression variable rewriting in subqueries in SPARQL parser.
Added support for SUM and AVG aggregates.