Merge pull request #1 from piskvorky/develop

catchup develop
'sample' parameter for downsampling frequent-words
simplify subsampling logic
word2vec: don't call random_sample when sample prob >= 1
Merge pull request #2 from piskvorky/word2vec_sample

Simplify subsampling logic
Merge pull request #206 from gojomo/develop

[word2vec] 'sample' parameter, for frequent-word down-sampling