new pydo introspection feature

In order to mitigate the performance hit of the table introspection
features in PyDO2, I've added a simple disk cache implementation that
can be turned on by setting MyObj.guesscache to True or to a
GuessCache instance.  This way, introspection will only need to be
done in one process, and later processes importing the class just need
to load a pickle.  To clear the cache, you just delete the pickle (1
pickle file per class).

This makes introspection quite a bit more palatable to me.

js

-- 
Jacob Smullyan

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