[MRG+2] 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.
Are you saying this because the row slice would be slower case of slicing the rows to calculate the mean? In that case I would explicitly convert it to `csc` for `manhattan` because of the median and to `csr` for the other metrics since it is easier to do the row slicing to calculate the mean.