Dan criteria instances

[dankohn]

@david-a-wheeler Please review this work to convert Criteria to a real class with an instance for each criterion. The key insight was from http://stackoverflow.com/a/36559091/1935918 which is that the class can and should instantiate if it is ever empty, such as on first use and when using reload! from rails console.

Regarding accessors, please note that you are currently testing for 3 accessors that are not present in criteria.yml. I dynamically generate the set of unique accessors and then manually add in those 3, plus :name.

Also note that I added memoization for most class methods via .each, which calls .all, which is memoized. I separately added memoization for .active and each use of .find_by_name.

Comments welcome as always. Otherwise, please mark that you approve and I'll squash merge.

[david-a-wheeler]

(Repeat of private email):
I wonder – would it be better to split this into 2 classes, “Criterion” to represent a single one, and “Criteria” to represent the collection of them all?

The “Criteria” class could be the place for queries like the badge level, as well as ways to get particular sets of Criteria (e.g., the “active” / non-future criteria).

[dankohn]

It seems like the class/instance method distinction perfectly matches the difference between Criteria and each criterion. Criteria.active correctly captures the appropriate subset, and memoizes the result for fast re-access. I'm not seeing the benefit of a separate Criterion class at this time (though we could always add it later if the need comes up).

[dankohn]

@david-a-wheeler Could I please get the OK to merge?

[david-a-wheeler]

Ok to merge!