Fix 619 by resolving redirects in SMWExportController

Returns a triple to point to the target with <swivt:redirectsTo rdf:resource= ... />
SQLStore / Fix subobject redirect access and id mismatch

- Fix null produced by SMWExportController::getSemanticData
- Fix id mismatch in SMWSQLStore3Readers::initSemanticDataCache for when subject has doesn't match
- Avoid redirect subobject resolve in SMWSql3StubSemanticData::getSubSemanticData
Merge pull request #622 from SemanticMediaWiki/rdf-redirects

Fix #619 by resolving redirects in SMWExportController