Add support for reverse_nested aggregations

[mausch]

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-reverse-nested-aggregation.html

I'm new to ElasticSearch so I'm just starting to learn things by using the Bloodhound types for guidance. I got a use case where apparently I need to use reverse nested aggregation but I notice it doesn't seem to be implemented in Bloodhound so I'm a bit lost about how it fits together with other search terms.

[bitemyapp]

https://github.com/bitemyapp/bloodhound#aggregations

https://github.com/bitemyapp/bloodhound/blob/master/Database/Bloodhound/Types.hs

Support for them would have to be added to the existing aggregations support. Aggregations haven't been released yet (next version will be 0.4.0.0), so churning the design is still in the cards.