fix DateHistogramFacet reference from 'zone' to 'time_zone' and add params

The `zone` parameter name in DateHistogramFacet is not recognized by elasticsearch.  This commit changes it to the recognized `time_zone`, and adds additional params per the elasticsearch documentation (http://www.elasticsearch.org/guide/reference/api/search/facets/date-histogram-facet/).  This also adds the new params to the `_serialize(self)` method so they are properly passed on to elasticsearch.