Ounit integration

I've been using this and thought I might contribute this back. It would be great to avoid the double printing of the name on failures but I'd like to see if it's useful for anyone else first.

We also need a local opam file to handle the depopts but it should be straightforward. 
I'd rather have this default value be `false`, so that cloning+compilating works even without `oUnit` (a bit unlikely, I agree).