Add simple script to verify presence of exercise directories

This is a very simple first step towards a solution to issue #6. Right now this works from x-api once you've init and update your submodules. It simply checks that each exercise listed has a corresponding directory present. If it doesn't it outputs some text to stderr and exits 1. I'm sure there is a lot more we can do here, but again, this might be a start. Let me know what you think.