Refactoring step invoker to avoid expose Mirror API

`MirrorList` is a internal class of Mirror API, and should not exposed in public VRaptor API. Many public methods returns or uses `MirrorList`, exposing our internal dependencies.

So I changed to `FluentIterable` to a better design. And `FluentIterable` can be easily updated to lambda expressions in the future, when we decide to use Java 8.
Because is used by an inner class (causes compilation error without final modifier).