[WIP] Add decision_function for OneVsOneClassifier.

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

This can't be correct since `self.estimators_` contains all one-vs-one classifiers... You need to aggregate the votes to form a `n_samples x n_classes` array. Have a look at how predict is implemented.