Enable golint & gofmt, resolve issues (#214)

* Add golint to linters and resolve issues.

I decided to remove constructors for some of the propagation types
because the constructors can be reduced to either using the zero value
or a single, non optional member.

* Enable gofmt and commit fixes

experimental/streaming/sdk/span_test.go

@@ -153,8 +153,8 @@ func measurementCompare(m1, m2 metric.Measurement) bool {
 
 func diffEvents(t *testing.T, got, want []exporter.Event, extraIgnoredFields ...string) bool {
 	ignoredPaths := map[string]struct{}{
-		"Sequence": struct{}{},
-		"Context":  struct{}{},
+		"Sequence": {},
+		"Context":  {},
 	}
 	for _, field := range extraIgnoredFields {
 		ignoredPaths[field] = struct{}{}