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 #384 from kmike/Nr-cache-fix

More efficient FreqDist._Nr_cache