Trace should provide a method to get active span like other language API does

[ghughal]

Java Tracer api has a way to get current active tracer.

https://github.com/opentracing/opentracing-java/blob/master/opentracing-api/src/main/java/io/opentracing/Tracer.java#L44

Javascript api should provide the same thing. Without that it becomes cumbersome to pass around root span throughout code base.