Add all ActiveRecord validations and callbacks to `RecordSpy`

This adds all of ActiveRecord's validations and callbacks to the
`RecordSpy`. This probably should've been done earlier, instead
of adding them as needed, but this should cover what ActiveRecord
currently provides.
Merge pull request #42 from redding/jcr-add-all-ar-validations-and-callbacks

Add all ActiveRecord validations and callbacks to `RecordSpy`