keyEvent integration test helper to simulate keydown, keypress etc.

keyEvent integration test helper in a similar vein to click, fillIn etc. to simulate keydown, keypress etc.

Example usage:

```
  visit('/posts').click('a:first', '#comments-link')
  .fillIn('.ember-text-field', "hello")
  .then(function() {
    equal(currentRoute, 'comments', "Successfully visited posts route");
    equal(Ember.$('.ember-text-field').val(), 'hello', "Fillin successfully works");
    find('.ember-text-field').one('keypress', function(e){
      equal(e.keyCode, 13, "keyevent chained with correct keyCode.");
    });
  })
  .keyEvent('.ember-text-field', 'keypress', 13)
  .visit('/posts')
  .then(function() {
    equal(currentRoute, 'posts', "Thens can also be chained to helpers");
  });
```