add support for classmethod decoration

[swayf]

added support for classmethods (needed last night :)

what do you think?

[mikeboers]

This is a tricky one.

I originally thought that swapping the order of the decorators would work:

@classmethod
@memo
def whatever(cls, ...):
    pass

and this does work for the immediate use case (i.e. returning memoized values), BUT it does not work with the methods on the memoized wrapper (e.g. exists) since they are not affected by the classmethod.

Looking at how you specifically implemented I feel that methods on metaclasses will get caught up, but I'm not too sure. I'm having too hard a time to wrap my head around the intricacies of descriptors today, apparently.

Perhaps the thing to do is to completely special case classmethod, and not try to be smarter about it.