[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!



Just a naive question, why do we import `libsvm` rather than have a pure cython version of one? 

Also if you decide that the `SVC` be fixed, I'll give it a shot?