Transition entire api to async

[cobbdb]

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

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

... or with normal callbacks ...

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

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

[cobbdb]

Would need to shim promise.. probably save that for a someday wishlist
http://caniuse.com/#feat=promises