Better path management - Changes in work section - Grunt workflow

* Changes made
 * Exclude from the `_config.yml` files that wouldn't be compiled and added to the `_site` folder.
 * Added `absolute_url` variable to `portfolio.yml` to better manage url through the entire template.
 * Images path changed, now the entire path is only inside `portfolio.yml`, this gives us more flexibility on the configurations.
 * Minor changes to the nested variables inside `.sass` files.
 * Big changes to the url building into `index.html`and `all.sass`. I've used the `prepend` [function](http://docs.shopify.com/themes/liquid-basics/output#prepend) 

Hope everybody will enjoy.