* Added new Zipkin tracing middleware using the native zipkin-go library
* new approach to Zipkin tracing middleware for Go kit
* improvements to zipkin tracer docs and code
* source comments changes
* Adds http test.
* Adds endpoint test.
* Improves code based on idiomatic feedback.
* updates to tracing doc comments
* do not allow to activate both zipkin-go and zipkin-go-opentracing to avoid confusion
* add test for zipkin http server tracer
* add test for zipkin grpc server and client tracers
