[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
Do you want me to refactor and place the code here, so that the code in the imputer can use this function? I haven't looked in detail at the imputer code, so I'm not sure.