Added "Second" facet date interval and support for custom date intervals...

Added "Second" facet date interval and support for custom date intervals on DateHistogramFacetDescriptor.
Elasticsearch now supports a constant value of "second" and custom time settings like "1.5h" on date histogram facet.
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-facets-date-histogram-facet.html#_interval