[MRG] Sparse One vs. Rest

This is a PR for the other parts in PR #2458 concerning sparse output support for the ovr classier. This branch was made off of the sprs-lbl-bin branch from PR #3203 and will be rebased to remove all the sparse label binarizer additions from the diff once the pull request is merged.
I would be interested in knowing if this is faster with `Y` in CSC. I expect that in CSR, `Y.getcol(i)` is a best-case O(nnz) operation; in CSC it is best case O(1).
If speed is a concern, though, I'd avoid `getcol`, use a CSC, and write a helper to fill a predefined array.