Throw an exception when the user attempt to create more than one instance of QApplication.
Throw an exception when the user attempt to create more than one instance of QApplication.

Reviewed by Marcelo Lira <marcelo.lira@openbossa.org>

enthought/pyside,gbaty/pyside2,IronManMark20/pyside2,PySide/PySide,pankajp/pyside,enthought/pyside,BadSingleton/pyside2,enthought/pyside,PySide/PySide,M4rtinK/pyside-bb10,RobinD42/pyside,M4rtinK/pyside-bb10,M4rtinK/pyside-android,gbaty/pyside2,enthought/pyside,qtproject/pyside-pyside,IronManMark20/pyside2,PySide/PySide,M4rtinK/pyside-android,qtproject/pyside-pyside,pankajp/pyside,BadSingleton/pyside2,IronManMark20/pyside2,BadSingleton/pyside2,pankajp/pyside,IronManMark20/pyside2,M4rtinK/pyside-android,RobinD42/pyside,RobinD42/pyside,pankajp/pyside,RobinD42/pyside,M4rtinK/pyside-bb10,gbaty/pyside2,qtproject/pyside-pyside,IronManMark20/pyside2,M4rtinK/pyside-android,RobinD42/pyside,M4rtinK/pyside-android,qtproject/pyside-pyside,M4rtinK/pyside-bb10,qtproject/pyside-pyside,gbaty/pyside2,RobinD42/pyside,PySide/PySide,BadSingleton/pyside2,pankajp/pyside,RobinD42/pyside,gbaty/pyside2,PySide/PySide,enthought/pyside,enthought/pyside,BadSingleton/pyside2,enthought/pyside,M4rtinK/pyside-bb10,M4rtinK/pyside-bb10,M4rtinK/pyside-android