[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.
This comment (the "as clusters") part is misleading.  I'd say "reassign at most .5 * batch_size"