create rating model

you can delete my ssok-rating-model branch too

Created rating model, 
Specifically the following files:
        new file:   app/models/rating.rb
	modified:   app/views/layouts/application.html.erb
	new file:   db/migrate/20141119015715_create_ratings.rb
	new file:   spec/factories/ratings.rb
	new file:   spec/models/rating_spec.rb