Track modified fields for ActiveRecord

[themightychris]

• Create a new state property for ActiveRecord like originalValues called modifiedValues, but that gets cleared upon success save
• Replace the use of originalValues with modifiedValues in isFieldDirty
• Store the contents of modifiedValues into a function-scope variable at the top of save, and pass it to the afterRecordSave event (since it won't be available within the model instance anymore at that point)