* Add a test for the behaviour of AttributeFactory.

* Don't crash when someone forgets to register an attribute.  Display a warning instead.

I had initially planned to put the warning in AttributeFactory::createAttribute(), but that resulted in a lot of noise since e.g. akonadiconsole uses unknown attributes.  So I put the warning when Entity::attribute() tries to cast it instead.

svn path=/trunk/KDE/kdepimlibs/; revision=973946