Customizable pluralize and singularize.

Companion of my latest Ember-Orbit pull request.
Thanks for getting started on `_findLink`! 

I think we need to avoid internally calling `find`, because links may be present at URLs that may be unique from resource URLs generated from `buildURL` (see [the JSON API spec](http://jsonapi.org/format/#urls-relationships)). In other words, we need a `buildLinkURL` method that easily be overridden, which can internally call `buildURL`.