add copyright notice to DownloadListener

app/src/main/java/org/schabi/newpipe/download/DownloadListener.java

@@ -9,6 +9,22 @@ import us.shandian.giga.get.DownloadMission.MissionListener;
 
 /**
  * Created by erwin on 06.11.16.
+ *
+ * Copyright (C) Christian Schabesberger 2016 <chris.schabesberger@mailbox.org>
+ * DownloadListener.java is part of NewPipe.
+ *
+ * NewPipe is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * NewPipe is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with NewPipe.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 class DownloadListener implements MissionListener