[WIP / RFC] Basic promise API support

Allows for things like:

```js
github.emoji().parsedBody.then(function(emoji) { console.log emoji })

Promise.all([
  github.emoji().parsedBody
  github.repo('groupon/gofer').contents().parsedBody
]).spread(function(emoji, rootContents) {
  // fun with emoji and the root contents of gofer
});
```