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. Or do something, when all components were initialised but were not started.

It can be done as follows:

```coffeescript
start: ->
  @mediator.user.fetch().then =>
    super
```