Support for Jekyll 1.0.0beta4

This makes the plugin runnable on Jekyll 1.0.0. The jekyll devs refactored the way the converters are initially configured ([see this commit](https://github.com/mojombo/jekyll/commit/3bc497c1c9fcffffc9e06099de541b8064d69494)). Default markdown converters are hard-coded now (jekyll throws an exception if we use an unofficial one), so I had to copy the whole `markdown.rb` file in order to make it work.