Serving static files when using the built-in webserver.

When using built-in webserver static files are not served, instead exceptions are thrown:
```
2013-09-18 14:43:29 Error: [Cake\Error\MissingControllerException] Controller class Css could not be found.
Exception Attributes: array (
  'class' => 'Css',
  'plugin' => NULL,
  'prefix' => NULL,
)
Request URL: /css/cake.generic.css
Stack Trace:
#0 /Users/renan/Sites/cakephp/3.0-app/webroot/index.php(41): Cake\Routing\Dispatcher->dispatch(Object(Cake\Network\Request), Object(Cake\Network\Response))
#1 {main}
2013-09-18 14:43:29 Error: [Cake\Error\MissingControllerException] Controller class Img could not be found.
Exception Attributes: array (
  'class' => 'Img',
  'plugin' => NULL,
  'prefix' => NULL,
)
Request URL: /img/cake.power.gif
Stack Trace:
#0 /Users/renan/Sites/cakephp/3.0-app/webroot/index.php(41): Cake\Routing\Dispatcher->dispatch(Object(Cake\Network\Request), Object(Cake\Network\Response))
#1 {main}
```

The current `AssetDispatcher` only tries to serve themed and plugins' assets.

From PHP's documentation:
> If a PHP file is given on the command line when the web server is started it is treated as a "router" script. The script is run at the start of each HTTP request. If this script returns FALSE, then the requested resource is returned as-is. Otherwise the script's output is returned to the browser.

In order to serve the static files from webroot directory it is necessary to `return false` on the router script and let the webserver serve it.