Added RangeAggregation
Added SignificantTermsAggregation
Add MetricsAggregationMixin and AvgAggregation.

The Metrics Aggregation Mixin can be used as the base
for most metrics based aggregations.  The AvgAggregation
uses the metrics aggregation mixin for all methods.
Added CardinalityAggregation
Added MaxAggregation
Added MinAggregation
Added PercentilesAggregation
Added StatsAggregation
Added SumAggregation
Added ValueCountAggregation
Added ExtendedStatsAggregation
Don't allow sub-aggregations for metrics aggregators.
Added DateHistogramAggregation
Added DateRangeAggregation
Added GeoDistanceAggregation
Added IPv4RangeAggregation