Provide ERC721 required functionality interface

contracts/token/ERC721.sol

+pragma solidity ^0.4.18;
+
+/**
+ * @title ERC721 interface
+ * @dev see https://github.com/ethereum/eips/issues/721
+ */
+contract ERC721 {
+  event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
+  event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
+
+  function balanceOf(address _owner) public view returns (uint256 _balance);
+  function ownerOf(uint256 _tokenId) public view returns (address _owner);
+  function transfer(address _to, uint256 _tokenId) public;
+  function approve(address _to, uint256 _tokenId) public;
+  function takeOwnership(uint256 _tokenId) public;
+}