[WIP] balanced accuracy score

As mentioned in #3506, I'm trying to implement balanced accuracy score.
Now it only supports binary classification.
Also, it is currently a prototype, which means I haven't worked on documentation and test. However, because this is the first time I contribute to scikit-learn, so I'd like to open the pull request first so that you can point out my mistake as early as possible. I also want to make sure my understanding to this issue is correct.

I'll keep working on this branch. Thanks.
That's supporting binary and multilabel, which I can easily motivate in terms of this metric. I don't know about multiclass; is there a reference paper describing balanced accuracy in a multiclass context.