No default connector/converter

This release is a minor release - removing the default values for connector and converter (which were previous set to Kafka and AVRO).

This is relevant because those classes depend on their specific classes which may be looked at during reflection and throw a ClassNotFoundException. For instance, if the connector is set to Kafka and the default config is loaded before the right value is set, the KafkaConnector will be looked up through reflection and try to load some Kafka classes which may not be on the classpath. This can happen with Kryo serialization, for instance.