fix error message in LidstoneProbDist
Remove GoodTuringProbDist (fix #381)
Remove references to GoodTuringProbDist
use SimpleGoodTuring properly in doctests (it had low performance because of unuseful default "bins" value)
strip extra whitespaces in nltk.probability
More efficient FreqDist._Nr_cache

FreqDist._Nr_cache is very sparse in real word. E.g. for Brown corpus there is only 553 non-zero values and 62160 zero values; for bigger corpora issue is worse, e.g. Russian Wikipedia has < 10K non-zero values and > 11M zero values.
much faster (and simpler) SimpleGoodTuringProbDist._r_Nr
fix SimpleGoodTuringProbDist._r_Nr for empty values
Merge pull request #382 from kmike/good-turing

Remove GoodTuringProbDist (see #381)
Merge pull request #384 from kmike/Nr-cache-fix

More efficient FreqDist._Nr_cache
Make DictionaryConditionalProbDist picklable. See #387
add a __reduce__ method to ConditionalFreqDist so that it's pickle-able
Failing test for #389
Merge pull request #389 from alexrudnick/fix_pickle_ConditionalFreqDist

add a __reduce__ method to ConditionalFreqDist so that it's pickle-able
Check that start is a Nonterminal in ContextFreeGrammar
Merge pull request #391 from onesandzeroes/master

Check the type of start in ContextFreeGrammar.__init__(): fixes #337
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