Render page content as markdown

* Installed Redcarpet Gem
* Add Helper to render text as HTML using Markdown Syntax 
  * Pages are written in Markdown, meaning they need to be converted to HTML when they are viewed.
  * This helper uses RedCarpet to parse the text its given and returns the generated HTML
  * It will be used like this in the views: `<%= @page post.body %>`