Implement the Evaluation techniques

[marcelcaraciolo]

Implement the recommender evaluation techniques: MAE, RMSE, F1-Score, Precision and Recall.

Evaluating collaborative filtering recommender systems References: http://dl.acm.org/citation.cfm?id=963772

Evaluation of recommender systems: new approach

http://ec.iem.cyut.edu.tw/drupal/sites/default/files/Evaluation%20of%20recommender%20systems%20A%20new%20approach.pdf

[marcelcaraciolo]

https://github.com/marcelcaraciolo/crab/blob/master/crab/evaluation/statistics.py https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/metrics/metrics.py

[marcelcaraciolo]

Added also the normalized mean absolute error NMAE.

[marcelcaraciolo]

Implemeted the metrics MAE, RMSE, F1-Score, Precision and Recall Check the Pull #57

[marcelcaraciolo]

Merged.