Serving static files when using the built-in webserver.

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.
Preventing directory traversal.
is_file() already checks if the file exists, no need for file_exists().
Merge pull request #4 from cakephp/3.0-webserver-fix

Serving static files when using the built-in webserver.