Mechanism to instantiate annotation for use with Guice's Key class

[jacospain]

For testing and for gradually retrofitting TypesafeConfig library into an existing codebase that has static or otherwise non-injectable config usages, it would be useful if there were an analog to Guice's built-in Names.named(String name) method to use in conjection with Guice's Key.get(Type type, Annotation annotation) method for querying the injector for specific named dependencies from the config.