Create test for User.get_rating_average().
Update README.

- Mention data structures for users, movies and ratings.
- Update documentation on User.ratings and Movie.ratings.
Merge branch 'master' of github.com:ntnu-smartmedia/elephant

# By Patrick Romstad
# Via Patrick Romstad
* 'master' of github.com:ntnu-smartmedia/elephant:
  Finished first version of simple kNN with cosine similarity between users and a prediction based on top 10 most similar users, their rating on movies and average rating for them