Revert "Remove unused callback traits"

This reverts commit b83d9c6.

include/tapkee/traits/callbacks_traits.hpp

@@ -0,0 +1,28 @@
+/* This software is distributed under BSD 3-clause license (see LICENSE file).
+ *
+ * Copyright (c) 2012-2013 Sergey Lisitsyn
+ */
+#pragma once
+
+namespace tapkee
+{
+
+template <class Callback> struct BatchCallbackTraits
+{
+    static const bool supports_batch;
+};
+#define TAPKEE_CALLBACK_SUPPORTS_BATCH(X) template <> const bool BatchCallbackTraits<X>::supports_batch = true;
+
+template <class T> class is_dummy
+{
+    typedef char yes;
+    typedef long no;
+
+    template <typename C> static yes dummy(typename C::dummy*);
+    template <typename C> static no dummy(...);
+
+  public:
+    static const bool value = (sizeof(dummy<T>(0)) == sizeof(yes));
+};
+
+} // namespace tapkee