Better path management - I'm sorry Travis

* 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. 
One more comment on that.

This is really great if our site is in a subdirectory ex.: www.domain.something/Subdirectory
If the theme is placed inside an url like that, all the url for the resource will not work. 

Instead if we add `absolute_url: /Subdirectory` or even better `absolute_url: www.domain.something/Subdirectory` all the linking process work through all the files. In the first method we have relative url settings, second one absolute.

I updated that because I used it to set up github pages on my repo: [rkpasia.github.io/Artists-Theme](rkpasia.github.io/Artists-Theme) __AS YOU CAN SEE THIS IS A SUBDIR ;)__