added ratings.json
nex version of rating. the API has changed, now the server should calculate the average rating and also determine whether the user has already rated a presentation or not.
added first test for RatingService
added reminding comment
added '/' + USER_TOKEN to the commented code to clarify my intention
Merge pull request #73 from SchweizerischeBundesbahnen/feature/mfr/rating

Feature/mfr/rating