Initial etckeeper formula for version control of /etc.

etckeeper is a simple wrapper around a VCS - git in our case - which keeps track of changes to the /etc dir.

This formula adds the etckeeper package, configures it to use Git, and initialises the repo.

The default package setup runs a commit after each package installation, and also one per day via cron.

This also adds hooks to Salt to commit at the start and end (via order:0 and order:last), to ensure changed made before and by Salt are recorded.

I did this originally, but it meant depending on not just the 'git' package -- it would bring in emacs-nox

Happy either way though, but considering salt allows multiple formula to depend on a package, this way is maybe cleaner (and allows sharing of the formula)