cleanup & refactoring

 - renamed "calc" aggregations to "metrics" aggregations
 - moved all bucket aggregation to a single "bucket" package
 - moved all metrics aggregations to a single "metrics" package
 - moved AggregatorParser to be an inner interface of Aggregator
 - move AggregatorFactory/ies to top level "aggregations" package
 - renamed "context" package to "support" package
 - cleaned up the test "aggregations" package