Doc2Vec defaults shouldn't override Word2Vec defaults without good reason

[gojomo]

Following #534, Word2Vec matches the word2vec.c defaults (as of the time Google stopped updating/maintaining that code). However, where our Doc2Vec class takes its own parameters – hs, negative, sample, window, and dm_mean – it's still using the older defaults. There's not really any case that these older defaults are better – and in some cases they're probably worse.

Unless there's a reason for different defaults, Doc2Vec should use the same values as Word2Vec... ideally by not specifying them at all (to avoid duplication). This probably means removing those params from the __init__() signature – but still documenting them in the doc-comment – so that the extra keywords just pass through to Word2Vec.__init__() if/when present.