Create camus-kafka-producer jar to minimize dependencies for producers

This change moves KafkaAvroMessageEncoder to it's own package with a minimum set of dependencies.  It no longer depends on avro-mapred and other dependencies from camus-etl-kafka.  Since it's interface uses types defined by Hadoop, it depends on hadoop-client now instead of hadoop-core.  Although hadoop-client itself depends on hadoop-core, it masks from unnecessary transitive dependencies such as jetty6.

All of this is an effort to eliminate conflicts that occur when embedded a Camus producer client into an application such as a RESTful API.