[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.

- [ ]  if the manhattan metric is used, does it necessarily mean the centroid should be the median? or should we have a separate keyword for the centroid alone.
extra =