Add python negative sampling

These modifications are mostly copied from @cod3licious 's wordvec
repository.
Initialize neu1e
Remove 'np.' and change index exclusion

Index exclusion now matches code in @cod3licious 's word2vec repository.
However, this differs from the cython version and from the original
word2vec implementation (or at least I think it does).

I don't know if we should exclude indices in word2_indices.
Merge pull request #4 from sebastien-j/negative

Negative