DATASOLR-121 - Add ability to add more group params.

Created GroupQuery for grouped solr queries.
Added queryForGroupPage to SolrOperations.
Implemented parse methods on ResultHelpers for SolrJ GroupCommands.
Added GroupQuery to documentation.

Original pull request: #44.
DATASOLR-83 - Add support for Realtime GET
Prepare issue branch.
DATASOLR-83 - Add support for Realtime GET
introduction of getById to SolrOperations;
documentation for real-time get.
DATASOLR-83 - Add support for Realtime GET

Use SolrRequest implementation for executing real time get request.
Added issue number to tests.
Added since tag to new methods.
Rebased on new master.