convert ERC721 to initializers

contracts/mocks/ERC721Mock.sol

 pragma solidity ^0.4.24;
 
+import "../Initializable.sol";
 import "../token/ERC721/ERC721.sol";
 
 
@@ -7,7 +8,11 @@ import "../token/ERC721/ERC721.sol";
  * @title ERC721Mock
  * This mock just provides a public mint and burn functions for testing purposes
  */
-contract ERC721Mock is ERC721 {
+contract ERC721Mock is Initializable, ERC721 {
+  constructor() public {
+    ERC721.initialize();
+  }
+
   function mint(address to, uint256 tokenId) public {
     _mint(to, tokenId);
   }

contracts/token/ERC721/ERC721.sol

 pragma solidity ^0.4.24;
 
+import "../../Initializable.sol";
 import "./IERC721.sol";
 import "./IERC721Receiver.sol";
 import "../../math/SafeMath.sol";
@@ -11,7 +12,7 @@ import "../../introspection/ERC165.sol";
  * @title ERC721 Non-Fungible Token Standard basic implementation
  * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
-contract ERC721 is ERC165, IERC721 {
+contract ERC721 is Initializable, ERC165, IERC721 {
 
   using SafeMath for uint256;
   using Address for address;
@@ -46,9 +47,12 @@ contract ERC721 is ERC165, IERC721 {
    *   bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))
    */
 
-  constructor()
+  function initialize()
     public
+    initializer
   {
+    ERC165.initialize();
+
     // register the supported interfaces to conform to ERC721 via ERC165
     _registerInterface(_InterfaceId_ERC721);
   }