Add reverse proxy call to owner

[frozeman]

This was proposed by @Amxx in this PR: https://github.com/ERC725Alliance/ERC725/pull/26 on the old code base.

owner might be a multisig, this allows read from/management of any eventual owning contract directly from the proxy. Usefull if the owning contract implements interfaces like ERC1271. That way dapps can call them without having to resolve the proxy ownership, thus making the proxy transparent

Very useful and should be discussed here.

This is especially relevant for the ERC725Account type (see types)

[frozeman]

       function fallback()
        public
        {
            address to = owner();
            assembly {
                calldatacopy(0, 0, calldatasize())
                let result := staticcall(gas(), to, 0, calldatasize(), 0, 0)
                returndatacopy(0, 0, returndatasize())
                switch result
                case 0  { revert (0, returndatasize()) }
                default { return (0, returndatasize()) }
            }
        }