[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
`X_sparse` should be renamed to something less ambigious. We typically use `X_something` to denote the dataset (i.e. `X_train`). Perhaps `is_X_sparse`