Move to organization go-nlp

[chewxy]

Move to https://github.com/go-nlp, which ideally will combine efforts of @james-bowman (TF-IDF, LDA, LSA etc) and @ynqa (Word Embeddings), possibly some help from @sajari guys (@dhowden, @mish15, @tbillington).

TODO

• Split out package corpus into go-nlp/corpus because the one thing that most NLP libs would require is a corpus, which is a map from string to int