Upgrade ownership of the contract: safe transfer of privileges

[sept-en]

The AdminControlled.sol contract now utilizes the improved ownership transfer as per #659, nevertheless, it's still possible to upgrade the admin to EOA/contract which doesn't own his key. In order to improve it, acceptAdmin() method should allow only the nominated admin to accept the ownership over the contract. This guarantees the nominated admin has control over his private key by the time of changing ownership of the contract.