Add Circle ci (#41)

* fix compile errors.

* fix lint errors.

* add circle-ci job.

* rename IDHigh to TraceIDHigh

* rename the file.

* add go get for goimports and golangci-lint

* enable GO111MODULE and remove comments.

* remove working dir and update cache name

* Add TEST_RESULT env back.

* run go mod tidy.

* remove go mod download.

* add test coverage.

* fix TraceID.

* fix circlefi config error.

* remove install-tools.

* remove ALL_TEST_SRC from Makefile.

api/trace/api_test.go

@@ -0,0 +1,28 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package trace
+
+import (
+	"testing"
+)
+
+// GlobalTracer return tracer registered with global registry.
+// If no tracer is registered then an instance of noop Tracer is returned.
+func TestGlobalTracer(t *testing.T) {
+	tracer := GlobalTracer()
+	if tracer == nil {
+		t.Fatalf("Failed to get tracer\n")
+	}
+}