Added code syntax highlighting

This is something I've wanted to see in the GitHub readmes for ages, and on technoweenie's suggestion I ported the syntax highlighting from Gollum to Markup, using Albino.

My changes also include a Processing class which makes it easy to add many more pre and post processing extensions to Markup e.g. to rewrite links or attributes, sanitise tags? etc.

I've also rewritten all the test examples to include rendering a snippet of Ruby code.