Added AliasPathStackResolver for name spacing paths to assets

Added a Path Stack Resolver that uses alias mappings.  This is good when you have a ton of modules installed and they all have a style.css file in their public/css/ directory causing a naming conflict between modules.  

To configure use the following config:

'asset_manager' => array(
    'resolver_configs' => array(
        'aliases' => array(
            'my/alias1/' => __DIR__ . '/../public/',
            'my/alias2/' => __DIR__ . '/../public2/',
        ),
    ),
),