Fix ConditionalFreqDist pickling so that it remembers how many times a sample happened, not just that it happened.
update doctest to show that we can pickle a ConditionalFreqDist and remember how many times a sample happened
suggestion from kmike to use a generator expression to avoid building the list in memory
Merge pull request #392 from alexrudnick/cfd_remember_how_many

ConditionalFreqDist should remember how many times each sample happened, not just that it happened