[FEATURE] Service#with_callbacks(prefix: nil) method added.

(taken from Kris Leech https://github.com/krisleech/wisper/wiki/Handle-events-with-private-or-protected-methods)

The method makes service's private methods accessible.
This allows service to listen to another one via protected/private callbacks.
Access can be restricted by a method's prefix.

== Addons

* Hexx::Service#with_callbacks.
* Hexx::Service::Callbacks module with this features defined.

== Changes

* Hexx::Service.params converted to public.

== Version

Release 2.1.0 launched.