Association tidying

While documenting associations, I found a few things to clean up:

- assocations implement `provideDefaults`, which returns a default options hash. This allows nicer inheritance of default options.
- move `::find` to Batman.Enumerable and take advantage of it in PluralAssociation

(Todo: this _can't_ remove the constant `|| Batman.currentApp`. Tests pass but that's a fluke. BRB)