Makefile: Have one so people know how to build this.

This produces a trivial Makefile that can run the app and also rebuild
from source, to simplify the process for people unfamiliar with javac
and jar commands.
Add Mac OS X guess at Plover configuration directory.

OS X default is ~/Library/Application Support/plover. Add check for
that so people don't need to manually enter the path in stenotray.cfg.