Add google prettify for code highlighting. Fixes #17

Adds google prettify to the theme.

I tested and works beautifully :)

To use, need to add ```prettify``` (and optionally the language name) to code blocks.

Example with Ruby:
![](http://couter.s3.amazonaws.com/snaps/Ghost_Admin_2014-07-19_21-49-01_2014-07-19_21-49-06.png)

Will display as:
![](http://couter.s3.amazonaws.com/snaps/Optimizing_Pumas_Backlog_for_Heroku_2014-07-19_21-50-21_2014-07-19_21-50-24.png)