pwkit/__init__.py: allow Holder to be used as a class decorator

This mode transforms the class definition into a Holder instance.
This can be convenient since sometimes you get annoyed typing

    mydata = Holder ()
    mydata.bar = 1
    mydata.foo = mydata.bar + 5
    ...

Thanks to the way that class declarations work, this decorator lets you you
get the same effect in a prefix-free namespace with a minimum of overhead.
pwkit/ellipses.py: note a useful reference