Add Role model.

Current spec status: 12 examples, 0 failures in 0.07942 seconds.
Add RolesController, stub #index action.

Current spec status: 14 examples, 0 failures in 0.27975 seconds.
Implement RolesController #new, #create, #show.

Current spec status: 27 examples, 0 failures in 0.12847 seconds.