Map metric

[beeva-ramiromanso]

From #58

Wanted to add the MAP metric to the evaluation.py script.

Same interface as mrr_score. If the training dataset is provided, scores of known interactions are set to very low values to discard them.

Next step: Tweak it to calculate MAP@K

[beeva-ramiromanso]

I'll try to create a small test for the metric. I have a controlled test with a set scoring instead of model.predict for the metric so I'll devise something.

As for the inline comments, sure, no problem.