Diverse Mini-batch Active Learning

[mbrine555]

This is a PR that implements a new batch active learning query strategy (as mentioned in #119). Diverse Mini-batch Active Learning attempts to take into account both informativeness and diversity when selecting a batch of new examples to be labeled. It's also worth noting that this involves bumping the required scikit-learn version from 0.18 -> 0.20.

I'm not sure if there's any additional documentation you'd like to have added around this, so just let me know!