Add Application#start, remove Application#startRouting.

Also, rename Application.initialized to .started for consistency.

`start` method is very useful in many apps.

You may want to pre-load some data before application is started.

It can be done as follows:

```coffeescript
started: ->
  @mediator.user.fetch().then =>
    super
```
Merge pull request #642 from chaplinjs/topics/application-start

Add Application#start, remove Application#startRouting.