Add @Generated annotation to all classes

It would be nice to annotate every generated class with `@Generated` annotation. This allows some IDEs to exclude these classes form certain code analysis. However, the jre standard `@Generated` was not introduced until java9 and Elemental2 is still supporting java8. In which case we could delay this change until java9 is the minimum supported version.

Alternatively, we could use the "enterprise java" `@javax.annotations.Generated` annotation but this would add a dependency solely for this annotation which seems sub-optimal. If we implement #25 and include `@javax.annotations.Nonnull` then the `@javax.annotations.Generated` annotation will be present in which case it would make sense to adopt this annotation for now.