Changes to recommenders API.

- Move SimpleKNN to recommenders package
- Add two API methods: add_users() and add_items(). Keep internal pointers to users and items.
  We use this API to support model-based recommenders in the future.
- Rename compute_recommendations() to get_recommendations()
Create movielens package.

Since the recommender algorithms are agnostic to which items they work on,
we make a separate package to contain the logic specific to the data.
This includes the parser, and models. The movielens package in turn implements
a simple API for working with the data.
Reflect architecture changes in main.py.
Merge branch 'master' of github.com:ntnu-smartmedia/elephant

# By Patrick Romstad
# Via Patrick Romstad
* 'master' of github.com:ntnu-smartmedia/elephant:
  Similarity metrics are now in a separate module
  Added Spearman's correlation coefficient
  Added Mean Squared Difference similarity measure
  Added descriptions to the functions

Conflicts:
	main.py
	src/recommenders/SimpleKNN.py