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.
Ohh, right. When develop the code I create an inner class inside the method. Before the test I extracted to external method, but missing the final. I'll remove later.