qualified dublin core to lobid conversion + update of lobid to oai_dc conversion

The qdc to lobid conversion introduces a modified version of NTripleEncoder. The NTripleEncoderWithSubject. The class allows the definition of an arbitrary but fixed subject. The DippQdcToLobidTest gives an example.

The oai_dc conversion is updated
- modifies OaiDcEncoder. Bug fix related to the closing oai_dc tag
- modifies OaiDcFlowTest. The test data comes now from api.lobid.org
- modifies NTripleDecoder. Bug fix. Literals are treated as literals now.