contract Ownable {
    address public owner;

    // Sets the sender address as the `owner`.
    function Ownable() internal {
        owner = msg.sender;
    }

    // Throws an exception if called by any account other than the `owner`.
    modifier onlyOwner() {
        require(msg.sender == owner);
        _;
    }
}

import './Ownable.sol';

contract MyContract is Ownable {
    function start() external onlyOwner {
        ...
    }

    function stop() external onlyOwner {
        ...
    }
}