add a __reduce__ method to ConditionalFreqDist so that it's pickle-able

At the moment, ConditionalFreqDist isn't pickle-able, but this patch makes it work with pickle.

This little script breaks when trying to unpickle on the current master, but works with the patch.

```python
import nltk
import pickle

cfd = nltk.probability.ConditionalFreqDist()
cfd['foo'].inc('hello')
with open("cfd.pickle", "wb") as outfile:
    pickle.dump(cfd, outfile)
with open("cfd.pickle", "rb") as infile:
    cfd2 = pickle.load(infile)
```