Bug 1027725 - Make calendar app async operations use promises

since we are repeating this logic everywhere what do you think about creating a new method?

```js
Calendar.denodeifyMethods(this, [
  'eventCapabilities',
  'getAccount',
  'findCalendars',
  'syncEvents',
  'ensureRecurrencesExpanded',
  'createEvent',
  'updateEvent'
]);
```

that way we avoid the `forEach` everywhere and possible errors.. -  to be honest I think the `forEach` is clean enough, just to see what you think about.