Remove mtl_firstObject, declare header so we can already use it in Xcode 4

As of iOS7, we don't need a custom firstObject accessor anymore and can use the method that was there all along. I've still added this to the headers so it will work with SDK6/Xcode4 as well.

Not sure how important backwards compatibility is here, I would simply drop this (or make mtl_firstObject that calls through to firstObject)