[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
yes, _get_median just has two arguments, `data` and the number of `non-zero`s, so I need to do the slicing here.