Ignoring deleted_at/deleted_on attributes

[pavelbabin]

I think it would be good to add to @ignored_attributes on Audited module attributes deleted_at and deleted_on by default.

Because it usually used by "paranoid" deletion gems like paranoia.

These attributes' values are setting up on destroy action. And no need to keep it in audits records like values for created_at/created_on attributes ignored on create action