Sql3StubSemanticData to load SubSemanticData on request

- Add '__sob' as datatype to identify subobjects
- Add `Sql3StubSemanticData::getSubSemanticData`
- Add `Sql3StubSemanticData::hasSubSemanticData`
Merge pull request #366 from SemanticMediaWiki/store-sobj

Sql3StubSemanticData to load SubSemanticData on request
Update RELEASE-NOTES.md

[skip ci]
Update BrowseBySubject to use make use of getSubSemanticData

- BrowseBySubject to rely on Store::getSubSemanticData
- Modify BrowseBySubjectTest to use \PHPUnit_Framework_TestCase
- Add BrowseBySubjectApiDBIntegrationTest