[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!



I found svm's `decision_function` which internally uses OvO, returns `n_classes*(n_classes-1)/2` decision values ( from all the classifiers ) per sample. Should we not implement something similar here? 
```
>>> x, y = np.random.randn(8,4), [1, 1, 2, 3, 3, 3, 4, 4]
>>> SVC().fit(x,y).decision_function(x).shape
(8, 6)
```