[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!



@amueller Could you check if this is correctly implemented? I am not getting the decision values to correspond to the predictions ( which if rightly implemented should ) or am I missing something?
I tried 
```
oc = OneVsOneClassifier(LinearSVC(random_state=0))
oc.fit(iris.data, iris.target)
oc.decision_function(iris.data).argmax(1) == oc.predict(iris.data)
``` 
The result array is only sparsely true... :/