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.
You sure?

```
$ cd Formula
$ find . -name '*-*' -exec grep class {} + | head -n 10
./activemq-cpp.rb:class ActivemqCpp < Formula
./adobe-air-sdk.rb:class AdobeAirSdk < Formula
./aircrack-ng.rb:class AircrackNg < Formula
./alpine-chappa.rb:class AlpineChappa < Formula
./android-ndk.rb:class AndroidNdk < Formula
./android-platform-tools.rb:class AndroidPlatformTools < Formula
./android-sdk.rb:class AndroidSdk < Formula
./ant-contrib.rb:class AntContrib < Formula
./apache-forrest.rb:class ApacheForrest < Formula
./apache-opennlp.rb:class ApacheOpennlp < Formula
```