Dissallow extending or implementing an unapproved types

[keith-turner]

The current code is ok extending an unapproved type as long as the members coming from the type are ok. While trying to deal with an Accumulo public API type that extends a non Accumulo public API type I realized that this should be prevented. Its awful.

[keith-turner]

Maybe its ok to extend classes that are not visible for sharing code (like extending a package private class). However extending visible unapproved types in the API seems problematic.