Update zipkin-receiver-kafka to use kafka 0.8.1.1

* updates kafka from 0.7 to 0.8.1.1
* fixes compile errors in zipkin-kafka 
* update zipkin-collector-service/src/main/scala/com/twitter/zipkin/collector/Main.scala to load zipkin-receiver-kafka instead of the scribe collector
* added in Span Decoder (serializer)
* fix up specs (still needs more fixup to test actual classes)