Make Word2vec multiple passes more explicit

[tmylk]

As discussed on the mailing list

Perhaps `train()` should require an explicit (non-default) `passes` parameter. In the 1-liner/initialization-trains case, it'd be called with the 'iter' value. For anyone calling it themselves, they'd have to make an explicit choice of 'passes'. And after the change, any old code without the parameter would (by design) break, forcing a change to explicit specification. 

Perhaps even `alpha` and `min_alpha` should be explicit required parameters to `train()`, to ensure those calling it directly aren't see-sawing the values each call. Calling 'train()' directly is kind of an advanced approach, so requiring this level of choice may be appropriate. 

[SND96]

@tmylk Would like to work on this issue.

[tmylk]

wip in #1139