Refactor the processor / validator / target / annotation model #282

I'm in the middle of a huge refactoring for #278, and I realize that it'd be nice to change AA internal architecture.

Currently, to support a new annotation, one has to create an ElementProcessor and an ElementValidator that both implement getTarget() from HasTarget, and then register them in AndroidAnnotationProcessor and register the annotation as well in the @SupportedAnnotationClasses annotation at the top level.

That's a lot of steps, and the really one thing I don't like is that all those parts are decoupled, although they are really related.

It should really be one annotation + one validator + one processor. And then registration of this tuple in only one place.

Side note : ElementProcessor is currently being removed, I'm replacing it with TypeElementProcessor and EnclosedELementProcessor.