Adds support for AS::Callbacks and smart deploy

This commit adds support for ActiveSupport::Callbacks to the
Paratrooper::Deploy class. All deploy methods trigger callbacks.

The Paratrooper Notifier concept has been refactored a bit to make use
of Callbacks. This was done without actually requiring any changes to
existing Notifiers that inherit from Paratrooper::Notifier.

An additional Paratrooper::Deploy option has been added to specify
callbacks directly. Callbacks differ from Notifiers only in that
they do not need to inherit from Notifier and can alter deploy
step execution.