Fix crash when the ListView has a header/footer.

If the ListView has a footer a ``ClassCastException`` is thrown

    E/AndroidRuntime(17730): java.lang.ClassCastException: android.widget.HeaderViewListAdapter cannot be cast to com.terlici.dragndroplist.DragNDropAdapter
    E/AndroidRuntime(17730): 	at com.terlici.dragndroplist.DragNDropListView.startDrag(DragNDropListView.java:163)
    E/AndroidRuntime(17730): 	at com.terlici.dragndroplist.DragNDropListView.onTouchEvent(DragNDropListView.java:127)
    E/AndroidRuntime(17730): 	at android.view.View.dispatchTouchEvent(View.java:7253)

This branch tries to address this problem.