Enabling saving/loading vocabulary alongside word2vec dump

Hello Radim,

Curious to know what you think about the following patch. Right now the vocabulary counts loaded from a word2vec model in gensim are misleading. This patch enables the output of SaveVocab() in word2vec to be used to restore the 'right' counts for words in the vocabulary. It also enables save_word2vec_format to persist a vocabulary file alongside the model (with real counts).

Best,
y