Convert CombinerParser.jj into freemarker template to allow custom parser implementations.

Hi Julian,

This is the change to allow Drill to extend CombinedParser.jj and add custom SQL statements, literals and data types. CombinedParser.jj and other codegen parts are packaged as part of the optiq-core.jar. 

Please have a look at it and let me know if any changes are needed.

Thanks
Venki