Update logtron to have no zookeeper

- Update latests kafka-logger
 - Change kafka interface to leafHost from host.
 - Add the defaultBackends feature to logtron
 - Test the defaultBackends
 - Actually have tests for kafka
 - Actually have a mock TCP KafkaServer
 - Update documentation.
 - Write a test to gaurd against circular objects.

cc @sh1mmer @kriskowal
We should make sure that the kafkaClient can be injected so that we can support both with and without Zookeeper kafka clients.