Prototype for 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 work on document and test. However, this is the first time I contribute to scikit-learn. Therefore, 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.