Add IsCallable method for Object in the API

include/v8.h

Index: include/v8.h
diff --git a/include/v8.h b/include/v8.h
index 5fc8059608d53adb47a4a393a1d0ceccd5a1bac1..457633a6e80f4f15226ea92ce519101c49c37987 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -1607,6 +1607,13 @@ class Object : public Value {
   V8EXPORT int GetIndexedPropertiesExternalArrayDataLength();
 
   /**
+   * Checks whether a callback is set by the
+   * ObjectTemplate::SetCallAsFunctionHandler method.
+   * When an Object is callable this method returns true.
+   */
+  V8EXPORT bool IsCallable();
+
+  /**
    * Call an Object as a function if a callback is set by the 
    * ObjectTemplate::SetCallAsFunctionHandler method.
    */