Create camus-kafka-coders jar to minimize dependencies for non-Hadoop contexts

This change moves the Kafka avro encoder and decoders to their 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.  Also, Camus consumers sometimes need the KafkaAvroMessageDecoder class to do message transformations outside of the CamusJob context.