Introduces a new UrlBuilder to create an article's public_url

Once again Goldencobra::Article doesn't need to know how to build a public url. There was too much logic inside article. This refactoring extracts the logic from Article and introduces it into a new class Goldencobra::UrlBuilder. Includes the unit tests for the new class.
Merge pull request #21 from 5minpause/refactor-new-url-builder

Introduces a new UrlBuilder to create an article's public_url