Refactor and add "exit" helper to base mixin
Package-updater scaffolding / proof of concept

* Added "Yum" namespace
* Created "yum" package-updater
* Created a generic script to dynamically validate, load, instantiate
  and invoke any package-updater
Rework gem structure

* Package-managers should live in the "Scriptilities::PackageManagers"
  namespace
* Add "Scriptilities::PackageManagers::Apt::Updater"
* Add "Scriptilities::PackageManagers::Brew::Updater"
* Rework "Scriptilities::PackageManagers::Yum::Updater"
* Bump gem version in preparation for a release