Added a "loader" method in Ember.Router to be used to load files async

Proposed fix for #2194

Use:

```javascript
App.Router.reopen({
      loader: function(handler, done){
        var modules = moduleName(handler.handler); //or a JSON map, or another method, up to developer to decide
        require(modules, function(module) {
          done();
        });
      }
    });
```

By default, it will call done() directly. So it is backwards compatible.