Data Model > Add didSet property observers to celeb's TotalVote and Ratings

[GreyEcologist]

• make sure totalVote is only incremented by one
• make sure ratings are not null, negative and within 1 to 5 ( make 1 and 5 class properties, see comment below)

[GreyEcologist]

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html