Support LOGIN authentication on outgoing SMTP server

PR:		#1
Obtained from:	https://gist.github.com/andelf/5118732

smtp.go

@@ -451,3 +451,30 @@ func validateLine(line string) error {
 	}
 	return nil
 }
+
+// LOGIN authentication
+type loginAuth struct {
+	username, password string
+}
+
+func LoginAuth(username, password string) smtp.Auth {
+	return &loginAuth{username, password}
+}
+
+func (a *loginAuth) Start(server *smtp.ServerInfo) (string, []byte, error) {
+	return "LOGIN", []byte{}, nil
+}
+
+func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
+	if more {
+		switch string(fromServer) {
+		case "Username:":
+			return []byte(a.username), nil
+		case "Password:":
+			return []byte(a.password), nil
+		default:
+			return nil, errors.New("Unkown fromServer")
+		}
+	}
+	return nil, nil
+}