Panic on invalid call

This pull request fixes two issues with the obj.Call() method:
 * Double call of the invoked method
 * Call() did not panic when the given method was not found.

I haven't run the test suite yet for the double call fix, since the test suite is broken under Mac right now (upstream problem with the packaging of QT in homebrew), so I suggest waiting until I did a quick run on my desktop at home until I can verify if it runs cleanly. Please also check for the C++ style.