[MRG] Fix MiniBatchKMeans 

This is a follow up on @amueller's PR:
https://github.com/scikit-learn/scikit-learn/pull/2638

The code was rebased on master

A minor test failure on my box was fixed

The strategy has been switched to uniform drawing of centers in the population.

@ogrisel, @amueller, please review. I believe that this should go in for 0.15.
Really "pick new centers". Sorry but the comments gave me a hard time understanding what the code does. Maybe it's just me, in the end it's all fairly standard.