Batman.Promise

I want to add Promises to batman.js.

(So far, I'm implementing Batman.Promise. I haven't applied it anywhere yet. Don't worry I'll yank out the `console.log`s once I don't need them anymore :P)

At first, I was shopping around to the different promise libraries, thinking we would need a "platform adapter"-type solution. But then I read the ["implementing"](https://www.promisejs.org/implementing/) page from Promises/A+. It seemed pretty easy actually, so I figured why not just but an implementation in batman.js? 

I aimed to match the ES6 Spec as closely as possible. Maybe someday you could just do 

```coffeescript 
if window.Promise?
  Batman.Promise = window.Promise
```

And delegate to the native implementation.