make httprespond an async helper

This works with the ember-testing-wait-hooks that have been introduced in ember 1.3.

Test can then be written this way:

```
visit('/login');
fillIn('.login-field-email', 'user@example.com');
fillIn('.login-field-password', 'secret-sauce');
click('.login-submit');
httpRespond('post', '/oauth/token', { authToken: 'your-token' });
andThen(function() {
  equal(currentURL(), '/success');
});
```