Support doc_values attributes on numeric and string mappings and in ElasticProperty attributes

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-core-types.html

Doc values are a new feature introduced in 1.0 to store field data in a column stride fashion. This allows for fast access to field data via fs cache without needing to put the field in the jvm heap.
Unit tests for doc values mapping via attributes and descriptor
tabs vs spaces
Merge pull request #644 from danp60/feature/doc_values

Support doc_values attributes on numeric and string mappings and in ElasticProperty attributes