Feature/nested destruction

The fix introduced in #38 for destruction of nested comments does not work for us. Two problems persist when deleting a record with a nested structure of comments:

1) not all comments are deleted: only the roots; direct children, grand children are not touched (see spec in attached PR)
2) with same structure, but creating comments through `Comment.build_from(... parent: abc)`: AACWT still tries to delete a non-existing comment

case 2 I failed to reproduce outside of our app. Are you interested?