add a __missing__ method so we don't have to hang on to a lambda, making ConditionalProbDist pickle-able
cleanup
make ConditionalProbDistI be a subclass of dict rather than defaultdict
add a test to show that ConditionalProbDist can be pickled
Merge pull request #394 from alexrudnick/pickle_ConditionalProbDist

Make ConditionalProbDist pickle-able by adding a __missing__ method