Allow using Maps to specify nested annotations

[Christopher-Chianelli]

All Java Annotations extend java.lang.annotation.Annotation,
which defines a method annotationType that return the type
of the annotation (and not the proxy class). As such, it can
be used as a key in a Map to specify the type of the Annotation
to create, with the remaining keys being the elements of the
Annotation.

[Ladicek]

I personally think that adding a method AnnotationCreator addNested(String annotationType) (that returns a new AnnotationCreator for the nested annotation) would be nicer, but this works too :-)