Support extended_bounds on Histogram

I have added support for [extended_bounds on histogram](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-aggregations-bucket-histogram-aggregation.html#search-aggregations-bucket-histogram-aggregation-extended-bounds), i hope this is a welcome contribution.
I'd prefer a strongly typed ExtendBounds object with `Min` and `Max` as properties.