DATASOLR-160 - Add Stats for Field
introduced equals and hashcode to SimpleField;
implemented StatsOptions for statistics request;
introduced Stats and SelectiveFacet annotations for field statistics
request;
introduced statsOptions to Query;
added statsOptions parsing on DefaultQueryParser;
introduced interpretation of Stats and SelectiveFacet to
SolrQueryMethod;
added result conversion for StatsFieldInfo from SolrJ to
spring-data-solr model;
added documentation for field stats;