Normalize newlines to '\n'

Change all \r\n into unix style newlines. Add missing newlines at the
end of a file and activate the newline checkstyle module, that enforces
'\n' as newline and a newline at the end of every file.

extensions/src/main/java/org/jivesoftware/smackx/muc/packet/GroupChatInvitation.java

@@ -109,4 +109,4 @@ public class GroupChatInvitation implements PacketExtension {
             return new GroupChatInvitation(roomAddress);
         }
     }
-}
+}

extensions/src/main/java/org/jivesoftware/smackx/muc/packet/MUCUser.java

@@ -621,4 +621,4 @@ public class MUCUser implements PacketExtension {
         }
 
     }
-}
+}