Added Aggregators for BloomFilter, CountMinSketch, HyperLogLog and Moments.
Changed HyperLogLog aggregator to produce the estimated size as final result.
Implemented aggregators for min and max.
Added aggregator for SketchMap.
* Changed HyperLogLog aggregator to return a HLL instead of the size.
* Fixed return types.
* Added aggregator that returns HLL size.
Merge pull request #194 from stephanh/more-aggregators

Added Aggregators for BloomFilter, CountMinSketch, MapSketch, HyperLogLog, Moments, Min, Max