Move ERC721 files to custom folder (#700)

contracts/mocks/ERC721TokenMock.sol

@@ -1,6 +1,6 @@
 pragma solidity ^0.4.18;
 
-import "../token/ERC721Token.sol";
+import "../token/ERC721/ERC721Token.sol";
 
 /**
  * @title ERC721TokenMock

contracts/token/ERC721/ERC721Token.sol

@@ -1,7 +1,7 @@
 pragma solidity ^0.4.18;
 
 import "./ERC721.sol";
-import "../math/SafeMath.sol";
+import "../../math/SafeMath.sol";
 
 /**
  * @title ERC721Token

test/token/ERC721/ERC721Token.test.js

@@ -1,4 +1,4 @@
-import assertRevert from '../helpers/assertRevert';
+import assertRevert from '../../helpers/assertRevert';
 const BigNumber = web3.BigNumber;
 const ERC721Token = artifacts.require('ERC721TokenMock.sol');