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.
Good idea! I just looked at how it was made in [date histogram](https://github.com/elasticsearch/elasticsearch-net/blob/337c5bae417e37303fab0834fe995396186195a4/src/Nest/DSL/Aggregations/DateHistogramAggregationDescriptor.cs#L56).

If i make a ExtendedBounds object, with two properties Min as Double and Max as Double, it's not posseible to reuse this object in DateHistogram, because it will be bounded to DateTime.

Maybe this is not a problem? What do you think?