This is a follow-up to PR #46. It replaces the 300ms loop with a file
watcher. Hawk is a wrapper around java.nio.file.WatchService coming with
proper OS X support.
Now rebuilds should be instantaneous.
This makes compilation of cryogen-core.watcher faster by reducing the
number of its dependencies. Now it depends just on pandect.algo.md5
instead of all pandect.algo.*