[MRG] Added metrics support for multiclass-multioutput classification

Fix for #3453 
Ping @arjoly . Added support for `zero_one_loss` and `accuracy_score`

Hmm. I didn't realise that this was a legal solution to the `if` continuation problem. I think I'd rather
```
            if merged_type.startswith('multilabel') or \
               merged_type == 'multiclass-multioutput':
```
but you can leave it as it is.