TLS for all

Added a new option WithTLSCredentials which allows ocagent to dial to servers using TLS Credentials.
This feature was requested in #44 and added with PR #45

This provides parity with ocagent's new TLS backed OpenCensus receiver with #45

Example

func Example_withTLS() {
	// Please take at look at:
	//    https://godoc.org/google.golang.org/grpc/credentials#TransportCredentials
	// for ways on how to initialize gRPC TransportCredentials.
	creds, err := credentials.NewClientTLSFromFile("my-cert.pem", "")
	if err != nil {
		log.Fatalf("Failed to create gRPC client TLS credentials: %v", err)
	}

	exp, err := ocagent.NewExporter(ocagent.WithTLSCredentials(creds), ocagent.WithServiceName("engine"))
	if err != nil {
		log.Fatalf("Failed to create the agent exporter: %v", err)
	}
	defer exp.Stop()

	// Now register it as a trace exporter.
	trace.RegisterExporter(exp)

	// Then use the OpenCensus tracing library, like we normally would.
	ctx, span := trace.StartSpan(context.Background(), "Securely-Talking-To-Agent-Span")
	defer span.End()

	for i := 0; i < 10; i++ {
		_, iSpan := trace.StartSpan(ctx, fmt.Sprintf("Sample-%d", i))
		<-time.After(6 * time.Millisecond)
		iSpan.End()
	}
}