Remove GoodTuringProbDist (see #381)

As @heatherleaf suggested, GoodTuringProbDist docstring is moved to a comment. I changed it a bit, please verify. 

Also, the default value for SimpleGoodTuringProbDist.bins is quite unuseful IMHO: freqdist.B() + 1 means that probability for unseen samples could be very large (up to several orders of magnitude larger than probability of the most common sample). Maybe there is a better estimate (e.g. could we somehow take Zipf law into account)?