class SimpleJob requires a valid object of class QCoreApplication

[Krzysztof Żelechowski]

If no valid application object is present, the constructor of SimpleJob causes 
a NULL pointer dereference.

Please add the following comment to the description of class SimpleJob:

An object of this class (or of a derived class) may only be created when the 
application object (derived from QCoreApplication) is present.

Thank you,
Chris