Utilise git submodules for external repos

[tschf]

Currently we are pulling needed files from raw.githubusercontent.com using curl (see scripts/ratom.sh). It might be neat to use git submodules and automatically pull sources through the built in functionality.

Useful resources:

• http://git-scm.com/book/en/v2/Git-Tools-Submodules
• http://stackoverflow.com/questions/3796927/how-to-git-clone-including-submodules
• https://chrisjean.com/git-submodules-adding-using-removing-and-updating/