Remove `allows`, `denies` and `denies_all` from Decorator

Automatic delegation of methods is now achieved with `delegate_all`,
which includes the new AutomaticDelegation module.

Manual delegation is achieved using the standard Active Support
`delegate` method, which is enhanced so that `to: :source` is the
default.
Merge pull request #419 from haines/remove_security

Remove `allows`, `denies` and `denies_all` from Decorator