Added --skip-tags option to ansible-playbook.

This adds a *--skip-tags* option to ansible-playbook.
It is identical to *--tags* except the behavior is of course the opposite: it will run all tasks who do NOT contain **some** tag that is in the provided list.

I know it is very desirable to have the docs updated, but I send this anyway because it is my first contribution to Ansible and any feedback is welcome :)