Transfer domains

[ilanolkies]

Enable to transfer different types of ownership.

Expected interface:

rns.transfer(domain, to) // if domain is uner .rsk, executes RNS Registry transfer and ERC-721 safe token transfer
rns.onlyTokenTransfer(domain, to) // executes .rsk token safe transfer, no registry transfer.
rns.onlyRegistryTransfer(domain, to) // executes RNS Registry setOwner method, no token transfer

Refs:

• https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/ERC721.sol#L177
• https://github.com/rnsdomains/rns-registry/blob/master/contracts/RNS.sol#L50