Added ReverseNestedAgg (which where missing).
Added test for NestedAgg and ReverseNestedAgg

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-reverse-nested-aggregation.html#search-aggregations-bucket-reverse-nested-aggregation
Merge pull request #425 from julianhille/feature/reverse_nested_agg

Added ReverseNestedAgg (which where missing).