Added article model to the anheimann_world project

Added the article model to anheimann_world application. The model contains the following fields: title:string, content:text, category_id:integer, active:boolean. In the app/model/article file the relationships with the category model was set (and in the app/model/category file the reverse relationship with article was specified), two scopes (alphabetical and active) were were set up, and the fields title and content were set up as required fields.