Add negative sampling (cython only)

Changes made to word2vec.pyx
word2vec.py has not beeen modified yet.
Not tested.
Merge branch 'w2v-negsam' of https://github.com/cod3licious/gensim into negative
Add random_numbers (array) to word2vec

For the Cython implementation of negative sampling
Minor bug fix
Correct syntaxt error in ctype def statement
Correct many mistakes

Now, the cython version assumes a table size of 10^8. Ideally, this
should be flexible. It might be advantageous to use a better random
number generator
Pseudo RNG closer to C version

Uses numpy random.randint at the beginning of each sentence, and then a
linear congruential generator (UNIX rand48, java).

Can deal with variable table size.
Add unit tests (plus minor changes)
Minor changes to formatting
Remove useless line
Fix RNG bug

Whenever target_index was equal to word_index, no new random number was
generated.
Compute g as with hierarchical softmax
Merge pull request #1 from sebastien-j/negative

Cython version