[MRG] Label power set multilabel classification strategy

Add one of the simplest and common multi-label classification strategy which use 
a multi-class classifier as a base estimator.

The core code is functional, but there is still things to do:
- [x] Add some word about binary relevance in ovr narrative doc
- [x] Write narrative doc about LP
- [x] Add some references  
- [x] Add some regression tests

- [ ] Explain that ovo and ovr are problem transformation method.
- [ ] "making your remark about overfitting a bit more explicit maybe?"
s/this have/this would have/ or /this has/
What's up with this section, is it really necessary? Is it for consistency with OvR?