Add support for scope manager

[mohabusama]

opentracing==2.0 introduced scope manager API. The tracer is responsible for scope_manager implementation.

The order for detecting parent spans needs to be updated. I suggest one of the following two orders:

Scope manager first

1. Using opentracing.tracer.active_span managed by the tracer context manager. The new span will be using the scope manager.
2. Using span_extractor.
3. Detecting span in kwargs.
4. Using call stack frames inspection.

Explicit first

1. Using span_extractor.
2. Detecting span in kwargs.
3. Using opentracing.tracer.active_span managed by the tracer context manager. The new span will be using the scope manager.
4. Using call stack frames inspection.

as span_extractor and passing spans in kwargs is more explicit.