Improved ERC721 granularity (#1304)

* Split enumerable and metadata implementations.

* Renamed ERC721Basic to ERC721, and ERC721 to ERC721Full.

* Fixed linter errors.

contracts/token/ERC721/IERC721Enumerable.sol

@@ -0,0 +1,21 @@
+pragma solidity ^0.4.24;
+
+import "./IERC721.sol";
+
+
+/**
+ * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
+ * @dev See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md
+ */
+contract IERC721Enumerable is IERC721 {
+  function totalSupply() public view returns (uint256);
+  function tokenOfOwnerByIndex(
+    address owner,
+    uint256 index
+  )
+    public
+    view
+    returns (uint256 tokenId);
+
+  function tokenByIndex(uint256 index) public view returns (uint256);
+}