Ruby Gem

I made a ruby gem using [Jeweler](https://github.com/technicalpickles/jeweler) since [Github Pages](https://help.github.com/articles/using-jekyll-with-pages) does not have support for plugins, only gems. The gem is published at [RubyGems.org](https://rubygems.org/gems/jekyll-prism-plugin).

Would be create to have a Gem for the script.