Restore .paranoid? method on all Mongoid Documents

In Mongoid 3, `.paranoid?` was defined on all Mongoid::Documents (regardless of whether or not they were paranoid).

When this gem was extracted, it was incorrectly changed so `.paranoid?` is only defined on Paranoid documents.

This PR restores the Mongoid 3 interface, i.e. `.paranoid?` is defined on all docs. This is more robust and will ease the migration path for Mongoid 3 users.