Issue #6. Change SubResource target to Class<?>

Change String to Class<?> annotation value.
This is obviously more typesafe, however way trickier to parse.

Annotation processor ain't allowed to parse class objects and
must rely on AnnotationMirror instances.

Based on this, I realized that unreachable classes will trigger
an early fail so I decided to remove my initial implementation
of ClassWorkLoad and fully rely on proper @SubResource parsing.