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.
Dang, i think I screwed up there :) I'll pull your PR as is and add a TODO to move both to a typed object in `NEST 2.0`. We can't change properties in minor releases do to strict semver requirements.