Refactor hook registration
Refactor hook registration

Rather than re-implement hook registration, lean on the existing methods
in Resque.

For example, here is how the `after_fork` hook is registered: https://github.com/resque/resque/blob/85706d536287910c53e0eb14e98c612b03dbea20/lib/resque.rb#L259

This fixes a bug where resque will error if a `before_child_exit` hook
has not been registered (https://github.com/stulentsev/resque-multi-job-forks/issues/19).

stulentsev/resque-multi-job-forks