Transition entire api to async

`defineSlot()` and `load()` will need callbacks. One option is to expose a promise:

``` javascript
harmony.defineSlot({..}).then(function (slot) {
    // do things with the slot.
});
```

... or with normal callbacks ...

``` javascript
harmony.defineSlot({..}, function (slot) {
    // do things with the slot.
});
```

The goal is to wrap the api inside of `googletag.cmd` for stability.
