Modernize dumpRDF script

- Fix `php SMW_dumpRDF.php` which just throws Filename cannot be empty in `SMW_ExportController.php`
- General script hygiene
- SMWExportController::printAll is now protected, the public interface
`printAllToOutput` and `printAllToFile` are two dedecated methods to do
only things they are responsible for
- Add tests for minimal test coverage