Limit lookup scope for private callbacks.

Background: Commit 71937c04ef introduced private callbacks, however, the use of "private_method_defined?" returns true all the way down the class hierarchy and thus causes trouble with a very common transition: Given an event "event :fail transitions_to :failed". If you don't explicitly define the "fail" event handler, "private_method_defined?" will still return true since Kernel#fail is defined for any object. Transitioning with "fail!" then causes a runtime error.
Merge branch 'master' of https://github.com/svoop/workflow into svoop-master
issue58: switch from private to protected callback methods