[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
Depending on how we handle my previous comment (about throwing an error on non-csc), you would want to check that it either works with a csr matrix or that it throws the error.