[#5] added Rating model
[#5] repr and unicode for Rating
[#5] tests for Rating.liked_text
[#5] tests for Rating init
[#5] tests for Rating unicode and repr
[#5] added 'popularity' field to Recipe; 'popularity' and 'slug' are now read-only in the Django admin
[#5] changed Rating.liked to Rating.vote; added one_or_negative_one validator
[#5] tests for one_or_negative_one
[#5] added Rating to admin