Add a classification accuracy objective

[dsherry]

Definition (from wikipedia):

This came up in the discussion around Dan Putler's initial work on a cross-platform performance comparison yesterday (notes in #293). Dan used a classification accuracy objective in Applied Modeling, H20 and R, but used recall in evalml. If we implement accuracy, that may provide a better performance comparison. Plus, the more metrics we support, the better.

It would be great to keep our implementation general enough to handle multiclass, but if we have to start with binary classification, so be it.

[kmax12]

related to this, we should also implement balanced accuracy

[angela97lin]

Got it, will update the objectives project plan w/ this!

[dsherry]

@angela97lin my suggestion is to list this issue in the future work section if you want, but to not allow it to affect the current objective API project scope. I think our priority should still be to finish the project; then this would be a great first issue to work on once we have it done. That sound ok?

[dsherry]

Ok after talking to Max and Angela, the plan is:

1. This is still separate from the objective API project
2. We can implement this at any point, before/during/after that project, which is why it's in the dev backlog. It's not a huge amount of work
3. Its low priority for now
4. This would be a good issue for a new hire