Only support after_initialize

Active Record doesn't support before_initialize, and it has issues with
attribute-not-found callbacks as it stands.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Use String#start_with? over String#starts_with?

The latter is a nice alias provided by Active Support, but we don't
require Active Support right now.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Reorder instance methods to reflect lifecycle

Save and update before destroy.

Signed-off-by: Stephen Celis <stephen@stephencelis.com>