most_similar_among functionality added with tests

[shubhvachher]

Fix #148. After @gojomo's comprehensive review from #1229

I have split PR's for all drive-by bug fixes as suggested. Some follow ups from previous comments :

Since people might not find index2word, I can see the benefit of an accessor method. However, given that this is now a generic KeyedVectors class, the name shouldn't be 'words'-centric. I'd suggest ordered_keys.

Done. I have also added a method to class Word2Vec in order to solve the original "people might not find index2word" problem.

I think suppress_warnings adds extra complication for little benefit.

Actually, calculating difference in sets (which is needed to generate this warning) can be an unnecessary overhead for people using this code in production environment. I have added a logging.info call to inform users of the same.
Sometimes it is not possible to remove all non-vocabulary words from the words-list, for example, if words-list is generated during runtime. Here using suppress_warnings would be ideal.

Seems more like something to log than use warnings.

Done. Yes, you're right. The module uses logging throughout, unnecessary import and use of warnings was not a good choice.

Because most_similar_among() functionality is merely on a set of vectors, it's not dependent on the mode-of-training – so there's no need for multiple models built different ways.

Ok sure, removing extra tests.

In fact, it'd ultimately make even more sense to be inside a set of KeyedVectors tests, rather than Word2Vec tests, but since its sibling methods are tested here it's not necessary to make that big change yet.

Quoted here to keep track.

[tmylk]

Please continue in the original PR