Improve set_perms_* performance

Setting perms on a large file tree takes forever, this improves the time drastically.

Example for directories on a Magento project:

Before:

    ubuntu@staging:/var/www/app/current$ time find /var/www/app/releases/20130718121318 -type d -exec chmod 755 {} \;

    real    0m34.930s
    user    0m2.792s
    sys 0m23.441s

After:

    ubuntu@staging:/var/www/app/current$ time find /var/www/app/releases/20130718121318 -type d -print0 | xargs -0 chmod 755

    real    0m0.264s
    user    0m0.052s
    sys 0m0.196s