Update the EstateOwner contract into an ERC721

[abarmat]

Convert the current EstateOwner implementation into an ERC721 compatible registry as it will make easier using the asset within the Marketplace and listing into other platforms.

• Refactor constructor
• Update single state variables to mappings accordingly in order to support multiple tokenIds
• Change current operator name into updateOperator or something meaningful
• Change EstateOwner to EstateRegistry (proposal)
• Update tests

Note: Use as a base for the ERC721 standard the OpenZeppelin implementation.

Reference: https://github.com/OpenZeppelin/openzeppelin-solidity/tree/master/contracts/token/ERC721