feat: indexed _tokenId on Transfer Event for ERC721 (#6)

contracts/token/ERC721/ERC721Basic.sol

@@ -6,7 +6,7 @@ pragma solidity ^0.4.21;
  * @dev see https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
  */
 contract ERC721Basic {
-  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
+  event Transfer(address indexed _from, address indexed _to, uint256 indexed _tokenId);
   event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
   event ApprovalForAll(address indexed _owner, address indexed _operator, bool _approved);