1) Adjust the GroupByQuery to also be able to merge results on the local node.  Fixes #116
2) Make the GroupByQuery operate in a multi-threaded fashion by default (this is configurable via druid.query.groupBy.singleThreaded).  Fixes #96
3) Fix up some post aggregation computation stuff.  I believe #72 is fixed
4) Fix case sensitivity issue with post aggregations on GroupBy queries