Introduce concept of TracerObserver

[objectiser]

Issues have been raised with two language clients that identify the requirement for a standard TracerObserver capability:

https://github.com/opentracing/opentracing-go/pull/135 https://github.com/opentracing/opentracing-java/issues/155

This issue should be used to capture the common requirements and define a high level approach that can be adopted by all OpenTracing languages.