[WIP] Add topics

This work started on the meetup along with @kamal-hothi.

Is this going to the expected way? Feedback, please! :smile: 

* [x] Topics have many questions. A question belongs to a topic;
* [x] Add shoulda-matchers to easier specs to validation, relations;
* [x] Add missing specs to validations and relations to Question;
* [x] Add basic administration to Topic on active admin;
* [x] Fix `db/seeds.rb` to add a "General" topic;
* [x] Relate questions and topics on [active admin](http://www.mickgardner.com/2012/12/habtm-and-activeadmin-forms.html);
* [ ] Modify questions views to relate topics;
* [ ] Add a topics#index view.