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.