add Initializable to simple ERC721 contracts

contracts/token/ERC721/ERC721Holder.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./IERC721Receiver.sol";
 
 
-contract ERC721Holder is IERC721Receiver {
+contract ERC721Holder is Initializable, IERC721Receiver {
   function onERC721Received(
     address,
     address,

contracts/token/ERC721/IERC721.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "../../introspection/IERC165.sol";
 
 
@@ -7,7 +8,7 @@ import "../../introspection/IERC165.sol";
  * @title ERC721 Non-Fungible Token Standard basic interface
  * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract IERC721 is IERC165 {
+contract IERC721 is Initializable, IERC165 {
 
   event Transfer(
     address indexed from,

contracts/token/ERC721/IERC721Enumerable.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./IERC721.sol";
 
 
@@ -7,7 +8,7 @@ import "./IERC721.sol";
  * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
  * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract IERC721Enumerable is IERC721 {
+contract IERC721Enumerable is Initializable, IERC721 {
   function totalSupply() public view returns (uint256);
   function tokenOfOwnerByIndex(
     address owner,

contracts/token/ERC721/IERC721Full.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./IERC721.sol";
 import "./IERC721Enumerable.sol";
 import "./IERC721Metadata.sol";
@@ -9,5 +10,5 @@ import "./IERC721Metadata.sol";
  * @title ERC-721 Non-Fungible Token Standard, full implementation interface
  * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract IERC721Full is IERC721, IERC721Enumerable, IERC721Metadata {
+contract IERC721Full is Initializable, IERC721, IERC721Enumerable, IERC721Metadata {
 }

contracts/token/ERC721/IERC721Metadata.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./IERC721.sol";
 
 
@@ -7,7 +8,7 @@ import "./IERC721.sol";
  * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
  * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract IERC721Metadata is IERC721 {
+contract IERC721Metadata is Initializable, IERC721 {
   function name() external view returns (string);
   function symbol() external view returns (string);
   function tokenURI(uint256 tokenId) public view returns (string);