Update BrowseBySubject to use #366

- BrowseBySubject to rely on Store::getSubSemanticData
- Modify BrowseBySubjectTest to use \PHPUnit_Framework_TestCase
- Add BrowseBySubjectApiDBIntegrationTest
Merge pull request #380 from SemanticMediaWiki/api-browse-subject

Update BrowseBySubject to use #366