improved exception handling in case of rollback to ensure that the original exception is not lost
changed logging to ensure that no messages are lost if the whole batch fails
added error handling for JSON encoding
added special handling for NAN values during JSON encoding
added type check
added DBpedia 100% benchmark for Stardog and Virtuoso (#51)
added graph composer archive to ignore list
removed timeout parameter as it is not recognized as integer in ini files; should be provided via YAML if required