Implement tokensByIndex extension

- Remove restrictions from mock mint and burn calls

contracts/mocks/ERC721BasicTokenMock.sol

@@ -12,6 +12,6 @@ contract ERC721BasicTokenMock is ERC721BasicToken {
   }
 
   function burn(uint256 _tokenId) public {
-    super.doBurn(_tokenId);
+    super.doBurn(ownerOf(_tokenId), _tokenId);
   }
 }