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. 
Ready to github pages.

Small setup guide:
* Create gh-pages branch in your repo.  __IMPORTANT: WORK WITH THIS BRANCH EMPTY__
* Open terminal and go to the cloned folder containing repo files.
* Inside this folder you absolutely have `_site` folder, right, go in here `cd _site`
* Now you are inside `_site` (in the terminal window of course) write `git init`. This set up an empty repo inside this folder.
* Use this command `git remote add origin <url_of_your_repo>`
* Now in your finder clean up all the files inside `_site`.
* Back to terminal `git fetch origin` this basically clone all the remote object (branches and others) to your `_site` local repository. 
* Type `git checkout gh-pages` and this command switch your HEAD to gh-pages branch.
* Now go up of one folder `cd ..` and run `jekyll build` 
* Come back to `_site` and use `git status`. Now you have all the compiled files to commit and push to gh-pages branch.

After the push go to devtips.github.io/Artists-Theme and you have the live preview of the template.