[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.
Just wondering: is this parameter needed, or could it be controlled through `reassignment_ratio=0`?

This could leave room for later adding `reassign_method={random, near, far...}` without needing to deprecate anything.