Support different writable modes

[gaambo]

PHP Deployer supports different writable_modes (chown, chgrp, chmod, acl, sticky, skip). At the moment we always run chown and chmod. In bfe5803 I added a check if chown is available before using it (so it can be disabled by setting writable_mode to anything but "chown"). But for other environments this may not be enough.
We've got a custom deploy:writable task, because we set the file permissions according to WordPress best practices, but we probably should support the different modes in the original deploy:writable from PHP Deployer.