Added rexmex to recommender systems.

[benedekrozemberczki]

Added RexMex - a recommender system evaluation library.

What is this Python project?

A recommender metric library for fair evaluation: covers coverage, classification, ranking, and rating.

What's the difference between this Python project and similar ones?

Comprehensive - it includes 30+ metrics that are not in other libraries.

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Anyone who agrees with this pull request could submit an Approve review to it.

[benedekrozemberczki]

@devinaconley, @zukijifukato, @Salv4d could you take a look into this?