cleanup, a fix, and a small new feature

I hadn't had a chance to use this plugin before, but I'm working on a consulting project where they want to add Facebook logins, so I've been having fun using someone else's Spring Security plugin. I did a little cleanup and since you're targeting Grails 2.4+ I converted what I could to use `@CompileStatic`. I found one bug - you need to add `def loadAfter = ['springSecurityCore']` in any addon plugin to be sure that plugin changes add to and override core settings. And I needed a small change in FacebookAuthProvider so I could override some behavior.