Bug 918970 - display existing thread when sending a sms from contact app

We now have a useful utility in utils: Utils.Promise.defer(). This helps reducing the indentation in the code that uses promises, and in the end it's usually more readable, so you can possibly have a look.

This is used like this:
```javascript
var defer = Utils.Promise.defer();
// defer contains 3 properties: `defer.promise` is the promise to return,
// `defer.resolve` and `defer.reject` are respectively the `resolve` and `reject` functions from the promise constructor

...
MessageManager.getMessages({
  filter: ...,
  each: ...,
  done: function() { defer.resolve(viewInfo); }
});

return defer.promise;
```

You see that the code has less indentation :)