add mintable erc721 token

contracts/token/ERC721/MintableERC721Token.sol

@@ -0,0 +1,13 @@
+pragma solidity ^0.4.21;
+
+import "./ERC721Token.sol";
+import "../../ownership/Ownable.sol";
+
+/**
+ * @title Mintable ERC721 Token
+ */
+contract MintableERC721Token is Ownable, ERC721Token {
+  function mint(address _to, uint256 _tokenId) onlyOwner public {
+    _mint(_to, _tokenId);
+  }
+}