Added additional nullable facts to DateEntry

If a facets date histogram query includes value_field or value_script then the additional json fields are included in the response. These are de-serialised as the facts: Total, TotalCount, Min, Max and Mean. They are nullable so do not impact current consumers of DateEntry.

http://www.elasticsearch.org/guide/reference/api/search/facets/date-histogram-facet/
Merge pull request #239 from CMXLabs/master

Add facts to DateEntry