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.
```
Error: problem in /usr/local/Library/Formula/signify-osx.rb
uninitialized constant SignifyOsx
Error: uninitialized constant SignifyOsx
```

Did you try it? `OSX` needs to be `Osx`.