Generating SPARQL grammar

By default, mvn compile just compiles pre-generated java sources.

To update java sources for SPARQL grammar, please follow next steps:

1. Make changes to src/main/java/com/bigdata/rdf/sail/sparql/ast/sparql.jjt
2. Run mvn generate-sources -P syntax. This step requires javacc 5.0 to be installed at /opt/javacc-5.0.
3. Commit changed files to git