ImportHandler should allow interfaces and abstract classes

[nlwillia]

It's not mentioned in 3.0 spec, but the API comment for ImportHandler#resolveClass says that it @throws ELException if the class is not public, or is abstract or is an interface., and implementations seem to enforce this. It may not be a best practice to reference static members via an interface, but it does happen, and with Java 8's added support for static methods on interfaces, this seemingly opinionated restriction should be removed from the API and left as a code style decision for the caller as it would be in the broader Java language.