Fixing Range Facets

 * Documents with unmapped key_field will be aggregated in a new "missing" count. Documents with a mapped key field but unmapped value field, will be aggregated into the "count" field of the appropriate range, but will not be taken into account in the value stats aggregations.

 * Fixed to properly compute both the doc count per range and the stats. Also added the following info:
   * top level "missing" - indicates the number of document that were missing a (key) value by which the ranges are matched
   * top level "other" - indicates the number of documents who's (key) values did not match any of the specified ranges
   * range level "missing" - indicates the number of documents that matched the range, but were missing a value for the stats calculation