Added option for alias in Router.

Just something i use a lot, because my models and controllers often share name.
So i added an alias option to router that redirect alias/* -> controller/*

If there already is a way to achieve this sorry for pull request. Just wanted to share this.

Example (application/config/routes.php):
$alias['page'] = 'page_controller';

will route all page/(all segments) requests to page_controller/(all segments)