Merge pull request #28 from alansaid/master

RMSE and NDCG
Re-designed, code checked

Added: more than one cutoff for NDCG.
Added: two types of NDCG computations (linear and exponential)
Re-designed, code checked

Added: more than one cutoff for NDCG.
Added: two types of NDCG computations (linear and exponential)
Merge remote-tracking branch 'upstream/master'