Allow for multiple hooks in after_prefork

Existing versions of Resque::Pool will only allow one caller to set the
`after_prefork` hook. When this is used for something like reestablishing the
ActiveRecord connection, it prevents other libraries from safely registering
their own hook.
Merge branch 'multiple_after_prefork_hooks' of https://github.com/jasonrclark/resque-pool into jasonrclark-multiple_after_prefork_hooks
preserve Principle of Least Surprise after_prefork= but also keep new multi-hook after_prefork behavior.