add signify

This is a port of the [signify(1)](http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man1/signify.1) utility from OpenBSD.
```ruby
test do
  system "#{bin}/signify", "-G","-n", "-p", "public.key", "-s", "secret.key"
end
```
or similar should do it. I've not tested this but just looked at the manpage.