Fix for asset pipeline (GPSHIRO-79)

When using the shiro-quick-start script the template for SecurityFilters.groovy does not work well with the asset pipeline plugin. All assets (images, css) are protected by shiro and you see broken images and no styles, unless you are logged in.

Adding a check for 'asset' controller fixes it.

You could arge that protecting assets are a feature that should be active by default but I think it causes more confusion than help for beginners. See https://jira.grails.org/browse/GPSHIRO-79