Add rubygem-protocol 1.0.1

This library offers an implementation of protocols against which you can check
the conformity of your classes or instances of your classes. They are a bit like
Java Interfaces, but as mixin modules they can also contain already implemented
methods. Additionally you can define preconditions/postconditions for methods
specified in a protocol.

WWW: http://flori.github.io/protocol/
WWW: https://github.com/flori/protocol