Update apt's cache only when repositories are added

This change makes running playbooks a lot faster especially when there
are no new repositories or packages. Updates and upgrades can be run
as pre tasks in playbook.
Fix file permissions in 'oh-my-zsh' role

File permissions were fixed by running tasks as the user they are being
installed for. Also made tasks a bit more readable.
Add defaults file for 'oh-my-zsh' role
Add 'php5-fpm' role

Tasks:
- Install PHP 5 FPM
- Install PHP 5 extensions (when defined)

Dependencies
- nginx
Add 'composer' role

Tasks:
- Install composer
- Install composer packages
Add 'php5-phalcon' role

Added tasks for installing requirements, getting source code and
installing and enabling Phalcon extension for PHP 5 FPM. Also handler
was added to 'php5-fpm' role to restart php5-fpm service.