[WIP] 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 have revised this line to sample its labels from the ```label_binarizer.classes_``` list and I wrote a test that uses string labels for the multiclass case.