Articles

PV: https://www.pivotaltracker.com/story/show/66011742
* Introduced `Article` model
* CRU actions for `Article`
* Added cukes and controller specs for `Article`
* Articles are written using markdown and supports syntax highlighting!
* Refactored some cuke steps

No view specs yet, too tired =( see PV: https://www.pivotaltracker.com/story/show/66268310