[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.
You don't need to use this function, especially if we don't add support to multi-label classification.