Result grouping - MBUnit & Gallio v3.4

Swapped unit test project Gallio 3.3 and MBUnit 3.3 references for
versions 3.4 in the Parl.SiteSearch SVN file system.
Hard-coded local Solr repository URL in IntegrationFixture.
Positioned data for testing of result grouping.
Added group by query functionality to GetGroupingQueryOptions in
SolrQueryExecuter.
Made GroupingParameters.Query an enumerable to cater for multiple query
parameters.
Extended GroupingResponseParser to parse group by query resultsets.