DATASOLR-65 - Added support for non-default request handlers.

Handler can be added to query via annotation:
@Query(requestHandler = "/instock")

pull request: #9
CLA: 46620130409043035 (Andrey Paramonov)
DATASOLR-66 - Upgrade to solr 4.2.1
[DATASOLR-65] - add docbox section covering request handlers
DATASOLR-42 - Add support for {!join