Add propagator interface and W3C propagator (#85)

* add propagation api.

* add http propagator interface and w3c propagator implementation.

* remove Extract api from trace.

* remove Extract interface for tracer.

* fix copyright.

* fix variable names and comments.

* move inject/extract out of trace.

* replace INVALID_SPAN_CONTEXT with EmptySpanContext function.

* fix tag.Map.

* make carrier as interface instead of http.Request.

* rename structs and update doc comments..

* add doc.go

* update doc.

* add noop propagator.

* add new propagation api with Supplier interface.
- added Default Tracer which simply propagates SpanContext.
- added CopyOfRemote option to simply create remote span.

* remove old propagator.

* rename propagator to TextFormatPropagator.

* rename default tracer/span as pass_through tracer/span.

* add test for pass through tracer.

* add missing interface to pass through tracer.

* return SpanContext instead of contex.Context from Extract interface.
- also remove PassThroughTracer

* fix review comments.

* add more test cases for traceContext extraction.

* remove tidy temporarily from circle-ci target to avoid build failure.

* allow header ending in dash '-'.

* add inject test for non-zero value other than 01 for traceoption

* add AddLink and Link interface to MockSpan

* fix running go mod tidy on every build.

api/trace/api.go

@@ -44,9 +44,6 @@ type Tracer interface {
 
 	// WithResources attaches resource attributes to the Tracer.
 	WithResources(res ...core.KeyValue) Tracer
-
-	// Note: see https://github.com/opentracing/opentracing-go/issues/127
-	Inject(context.Context, Span, Injector)
 }
 
 type FinishOptions struct {
@@ -102,13 +99,6 @@ type Span interface {
 	ModifyAttributes(...tag.Mutator)
 }
 
-type Injector interface {
-	// Inject serializes span context and tag.Map and inserts them in to
-	// carrier associated with the injector. For example in case of http request,
-	// span context could added to the request (carrier) as W3C Trace context header.
-	Inject(core.SpanContext, tag.Map)
-}
-
 // SpanOption apply changes to SpanOptions.
 type SpanOption func(*SpanOptions)
 
@@ -157,18 +147,6 @@ func Start(ctx context.Context, name string, opts ...SpanOption) (context.Contex
 	return GlobalTracer().Start(ctx, name, opts...)
 }
 
-// Inject is convenient function to inject current span context using injector.
-// Injector is expected to serialize span context and inject it in to a carrier.
-// An example of a carrier is http request.
-func Inject(ctx context.Context, injector Injector) {
-	span := CurrentSpan(ctx)
-	if span == nil {
-		return
-	}
-
-	span.Tracer().Inject(ctx, span, injector)
-}
-
 // WithStartTime sets the start time of the span to provided time t, when it is started.
 // In absensce of this option, wall clock time is used as start time.
 // This option is typically used when starting of the span is delayed.