Initial commit of simple CI code

I have all the bits needed for a simplified distributed CI, but it doesn't yet run the tests. I have a few more hours work to get everything running as expected, but I thought I should check in some code by now.
You don't need to check whether .commit_hash exists if you're going to 'rm -f' it; if the file doesn't exist, -f won't emit an error message and will exit successfully (status 0).