Add ActiveRecord spy for testing mixins and configuration

This adds a `RecordSpy` for testing ActiveRecord mixins. This
is a useful tool for testing mixins that add ActiveRecord
validations, callbacks, etc. The goal is to be able to test these
mixins without actually providing an ActiveRecord model.
Merge pull request #39 from redding/jcr-configuration-spy

Add ActiveRecord spy for testing mixins and configuration