Less obtrusive addition of test hooks
Generalized method for test hooks
Move hook testing stuff to Test::Qpsmtpd

This makes it more available to plugins outside of t/plugin_tests/
Add some comments on uses for fake_{hook,config}
Take advantage of fake_hook() in existing tests
More explicit explanations
Merge pull request #169 from jaredj/hook-testing

Generalize hook testing
Add testing for greylist()
More greylisting tests