Adds syntax highlighting to Casper via Prism

This commit adds two files to Casper: css/prism.css and javascript/prism.js. This comit also adds corresponding changes to  default.hbs. 

I chose Prism for the syntaxing highlighting because it is a lightweight and modern highlighter. The minified version of prism.js includes all language options that are available on the following page: 
http://prismjs.com/download.html