Add to Ownable a function to renounce ownership

[frangio]

I think we should have a function in Ownable to set the owner state variable to 0, since this can't be done by using transferOwnership.

Not sure about the name, but I was thinking renounceOwnership.

[OTTTO]

Seems superfluous.
Owner can just call the function and set the new owner to 0x0..., any listeners would than receive this information from the event. And anyone who wants to verify for themselves can simply check the public owner variable.

[frangio]

@OTTTO, in fact transferOwnership cannot be used to set the owner to 0x0 since a while ago. Hence this issue.

https://github.com/OpenZeppelin/zeppelin-solidity/blob/77cc33fc5c4d0b0d03aec1d8bba77edc9a7196ae/contracts/ownership/Ownable.sol#L37

[stefek99]

I saw the updated code and I agree with @OTTTO that it seems superfluous - had to Google that word.

There is require(newOwner != address(0)); preventing from accidental disasters, alternative approach would be to use so magic value 0x00000000001 here, so the renounce makes more sense.