Don't start the meat of the app until the inspected page is ready

The inspector reloads the page with the backboneAgent injected, so we
need to wait until the page is re-loaded to begin our inspector app.
I hate this boiler plate... can we make a generic `eventToPromise`. We can keep this in `utils`

```js
this.pageReady = eventToPromise(this.inspectedPage, 'ready');
```

eventToPromise: function(eventable, event) {
  var res;
  var promise = new Promise(function (resolve) {
    res = resolve;
  });
  eventable.on(event', res);
  return promise;
}
```