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.

Original Pull Request: #46
DATASOLR-160 - Polishing.

Added issue numbers and author information.
Moved and renamed tests.
DATASOLR-214 - Fix test leaving index files behind.

We new explicitly remove the data directory using a shutdown hook.