Dynamically rebind 'after rubber:install_packages' callbacks to 'after rubber:setup_volumes' during bootstrap to avoid duplicating work if an attached volume would contain the deployed application.
Added a newline for legibility.
Reworked the internals on installing packages without callbacks before volumes are set up.

The core idea here is the same, but rather than mess around with Capistrano's internals, a new doppelganger task was created to do virtually the same thing as :install_packages, but without the callbacks.
Merge pull request #451 from rubber/rebind_after_install_packages_callbacks

Dynamically rebind 'after rubber:install_packages' callbacks to 'after rubber:setup_volumes' during bootstrap to avoid duplicating work if an attached volume would contain the deployed application.