Issue #204

A reasonable approximation of the method described in the paper Distributed Representations of Sentences and Documents (http://cs.stanford.edu/~quocle/paragraph_vector.pdf).

Python isn't my first language, so I don't pretend that I did everything in the most "pythonic" way here, but I pulled out the portions of the word2vec code that needed to be modified into their own methods, then added another class extending word2vec which just modifies those few functions.

I also don't really know what to do in terms of refactoring cython code to limit code duplication, so doc2vec has its own set of cython functions which are completely independent of the word2vec ones.

Hope that helps.
Tim