Allow disabling of login and token validation

In my project, I'm using the plugin for API access using fixed access tokens, so I don't need explicit login, logout and validation endpoints. The pull request adds configuration options ```grails.plugin.springsecurity.rest.login.enabled``` and ```grails.plugin.springsecurity.rest.token.validation.enabled```, which allow to disable those. It's a minor change and would really improve the versatility of the plugin.

If the changes are generally okay, I'd add some information to the docs and update the pull request accordingly.