[WIP] Kill submodules

This PR removes the use of git submodules for managing external Ansible role dependencies. Instead it uses Ansible [role requirements files](http://docs.ansible.com/galaxy.html#advanced-control-over-role-requirements-files) introduced in Ansible 1.8. Since Ansible 1.8 is not yet released, please **do not merge this PR** yet.

I've tested the functionality in this PR by using the [latest Ansible source](https://github.com/ansible/ansible) which contains the necessary 1.8 functionality for role requirements files.

Fixes #753.
:thumbsup: excellent.

Should we `.gitignore` the role directories that are specified by the requirements file, too?