[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 this with np.average.