move Ack to top level

src/main/java/com/github/nkzawa/socketio/client/Ack.java

@@ -0,0 +1,11 @@
+package com.github.nkzawa.socketio.client;
+
+/**
+ * Acknowledgement.
+ */
+public interface Ack {
+
+    public void call(Object... args);
+
+}
+

src/main/java/com/github/nkzawa/socketio/client/Socket.java

@@ -356,14 +356,5 @@ public class Socket extends Emitter {
         }
         return data;
     }
-
-
-    /**
-     * Acknowledgement.
-     */
-    public static interface Ack {
-
-        public void call(Object... args);
-    }
 }