[MRG] ENH: Patches Nearest Centroid for metric=manhattan for sparse and dense data

Fixes https://github.com/scikit-learn/scikit-learn/issues/743

I have also added a utility for calculating the median of csc sparse matrices, since NumPy does not handle it and it is not a trivial one-liner
Instead of relying of _get_median, I was talking about the possibility to have only sparse median everywhere?
I would for instance avoid circular import. ;-) Maybe get_median should be in this file.