Add feature encoding algorithms (Fisher vector, VLAD, and BoW)

[DavidTorpey]

I was wondering if it would be possible to include the three main popular feature encoding / pooling / quantisation algorithms into scikit-learn, namely, Fisher vectors, VLAD, and BoW. They are very popular, time-tested algorithms with many applications.

I am happy to submit this feature as well.

Originally proposed for scikit-learn, and was suggested instead to go into this library:
scikit-learn/scikit-learn#14382

[adrinjalali]

Hmm, I think the idea here was to include methods which don't pass the inclusion criteria, but the issue with these methods is the API.

I think it wouldn't hurt to be a bit more liberal with the API in this repo, but not sure.