Make DefaultPrettyPrinter.createInstance() to fail for sub-classes

[cowtowncoder]

Pattern of "blueprint object" (that is, having an instance not used as-is, but that has factory method for creating actual instance) is used by Jackson in couple of places; often for things that implement Instantiatable. But one problem is that unless method is left abstract, sub-classing can be problematic -- if sub-class does not override method, then calls will result in an instance of wrong type being created.

And this is what can easily happen with DefaultPrettyPrinter.

A simple solution is for base class to make explicit that if base implementation is called, then instance can not be a sub-class (that is, it is only legal to call on DefaultPrettyPrinter, but no sub-class). This is not optimal (ideally check would be done compile-time), but better than getting a mysterious failure.