Metric runner: fixed #31. And clean-ups

TODO: test the new runners
Merge branch 'master' of https://github.com/recommenders/rival

Conflicts:

	rival-evaluate/src/main/java/net/recommenders/rival/evaluation/metric/error/MAE.java

	rival-evaluate/src/main/java/net/recommenders/rival/evaluation/metric/error/RMSE.java