The previous method was to change self.__class__ dynamically to make __init__
instantiate different classes. This new way, which uses bare functions instead
of separate functor-like classes, instead just forwards the resize() call to
a module-global implementation based on self.method.
Additionally, the semantics of ArtResizer have changed. Clients now *always*
call resize() and proxy_url(), regardless of method. The method makes *one* of
these a no-op. This way, clients need not manually inspect which method is
being used.