Create Ember.Test with registerHelper method

This will expose an `Ember.Test` that will allow us to expose methods for users to customize their testing environment.

The first thing I added was `Ember.Test.registerHelper` method, which allows us to create our own custom helpers.

The helper will always get passed the current application as a first argument:

```javascript
Em.Test.registerHelper('boot', function(app) {
  Em.run(app, app.advanceReadiness);
  return wait();
});

App = Em.Application.create();
App.setupForTesting();
App.injectTestHelpers();

boot().then(function() {
  return visit('/posts');
}).then(function() {
  // assertion
});
```

It's very easy to create async helpers, all they need to do is `return wait();` at the end of the helper.


The second thing I added is the ability to register callbacks that would be fired when `App.injectTestHelpers` is called.

```javascript
Ember.Test.onInjectHelpers(function(app) {
  console.log('helpers where injected into ' + app);
});
```