* Add new propagators package
* Move B3 propagator to propagators
Update all `api/trace` dependencies in the propagator.
Export the isDeferred and isDebug methods of the SpanContext. These are
needed by the B3 propagator to track trace state.
* Move W3C trace context propagator to propagators
* Update package docs with supported encodings
* Move unified propagators code to own file
* Update b3 exported documentation
* Update trace_context exported documentation
* Add code examples for B3 propagator
* Add TraceContext example code
* Remove internal package
Move common testing declarations to the propagators_test.go file.
* Add changes to Changelog
* Add test to check default propagators
