[WIP] Add decision_function for OneVsOneClassifier.

Adding `decision_function` support for the `OneVsOneClassifier`.

This is based off of #1548 and addresses #1523.

##### Todo:

- [ ] Correctly implement the `decision_function` for `OneVsOneClassifier`
- [ ] Correct the tests
- [ ] Incorporate the `decision_function` output in `test_ovo_ties`

@arjoly, @amueller, @mblondel  Pl. review when you find time!



Ah! So I was working with the outputs of all the n*n-1/2 classifiers... 
BTW, do I return the votes or the scores as decisions? If I return scores, I get a 60% match ( which is fair for aggregated scores ) and so the `assert_almost_equal` does not pass...