DATASOLR-83 - Add support for Realtime GET

Introduction of getById to SolrOperations;
Documentation for real-time get.

Original Pull Request: #45
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.
DATASOLR-83 - Add support for Realtime GET.

Introduced some tests for SolrRealtimeGetRequest.