Adding possibility for callback value registraion with Spark::function

...allowing implementations of function calls to class members, should resolve #313

Allows following pattern as mentioned by @m-mcgowan on this thread: http://community.spark.io/t/how-to-point-spark-function-to-a-class-function/6986/4

```cpp
class MyClass {
   static int callback(String arg, void* pv) {
       return ((MyClass*)pv)->handle(arg);
   }

   virtual int handle(String arg) {
       return 42;
   }
}

MyClass my_class;
Spark.function("myfunc", MyClass::callback, &my_class);
```