Added the ability to chain test helpers

This gives us the ability to chain helpers:

```javascript
test("Adding a post", function() {

  visit('posts/new')
  .fillIn('.txt-title', 'My Post')
  .fillIn('.txt-body', 'This is the body')
  .click('.btn-submit')
  .then(function() {
    equal(find('.post-title').text(), 'My Post');
    equal(find('.post-body').text(), 'This is the body');
  })
  .click('.post-title')
  .click('.btn-new-comment')
  .fillIn('.txt-comment-body', 'My Comment')
  .click('.submit')
  .then(function() {
    equal(find('.comment').length, 1);
  });

});
```

Any custom helpers added using `Ember.Test.registerHelper` method would also be chained:

```javascript
Ember.Test.registerHelper('boot', function(app) {
  Em.run(app, app.advanceReadiness);
  return wait();
});

Ember.Test.registerHelper('reset', function(app) {
  app.reset();
  return wait();
});

// ... later in the test
boot().visit('posts').click('.post-name').reset();

```