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.

plugin/httptrace/api.go

@@ -18,12 +18,10 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptrace"
-
-	"go.opentelemetry.io/api/trace"
 )
 
 // Client
-func W3C(ctx context.Context, req *http.Request) (context.Context, *http.Request, trace.Injector) {
+func W3C(ctx context.Context, req *http.Request) (context.Context, *http.Request) {
 	t := newClientTracer(ctx)
 
 	t.GetConn = t.getConn
@@ -45,5 +43,5 @@ func W3C(ctx context.Context, req *http.Request) (context.Context, *http.Request
 
 	ctx = httptrace.WithClientTrace(ctx, &t.ClientTrace)
 	req = req.WithContext(ctx)
-	return ctx, req, hinjector{req}
+	return ctx, req
 }