feat: add NonceTracker

contracts/access/ERC721Minter.sol

@@ -2,6 +2,7 @@ pragma solidity ^0.4.23;
 
 import "../token/ERC721/MintableERC721Token.sol";
 import "./SignatureBouncer.sol";
+import "./NonceTracker.sol";
 
 /**
  * @title ERC721Minter
@@ -14,7 +15,7 @@ import "./SignatureBouncer.sol";
  * @dev 4. Generate a valid bouncer signature and submit it to the ERC721Minter using `mint`
  * @dev Override `mint()` to add tokenURI information.
  */
-contract ERC721Minter is SignatureBouncer {
+contract ERC721Minter is SignatureBouncer, NonceTracker {
   MintableERC721Token public token;
 
   constructor(MintableERC721Token _token)
@@ -25,6 +26,7 @@ contract ERC721Minter is SignatureBouncer {
 
   function mint(bytes _sig)
     onlyValidSignature(_sig)
+    withAccess(msg.sender, 1)
     public
     returns (uint256)
   {

contracts/access/NonceTracker.sol

@@ -0,0 +1,30 @@
+pragma solidity ^0.4.23;
+
+
+contract NonceTracker {
+  mapping(address => uint256) private nonces;
+
+  modifier withAccess(address _address, uint256 _nonce)
+  {
+    access(_address, _nonce);
+    _;
+  }
+
+  function nonce(address _address)
+    public
+    returns (uint256)
+  {
+    return nonces[_address];
+  }
+
+  /**
+   * @dev call this function when accepting a nonce
+   * @dev throws if nonce is not strictly greater than previous nonce
+   */
+  function access(address _address, uint256 _nonce)
+    internal
+  {
+    require(_nonce > nonces[_address]);
+    nonces[_address] = _nonce;
+  }
+}