Add evaluator class

[paxcema]

Up for discussion.

A stateful class could help enabling the following:

• Track evaluations through time in a simple way
• Store multiple-metric evals in a single object (users may want to eval a model with multiple metrics and consolidate all of these in a single artifact, which would be easier to produce from a single object)
• Focus on reproducibility: there should be a simple mechanism that enables automatic logging of everything that is done to/with a marked instance
• Scheduled support (i.e. cron jobs)? Not 100% sold on this one, it may be better to implement it within mdb proper