Add support for ERC-721 tokens

[acravenho]

As reported by @alexgaribay this week some tokens do not have total supply and other functions that other ERC-20 tokens usually possess. Most likely these token transfers are ERC-721. There are a few slight differences in these 2 types of token transfers.

ERC-721 are non-fungible tokens and every token can be owned by a separate owner.

1. ERC-721 have a 4th topic which is the TokenID of that particular token being transferred
2. The data field of the token which usually determines the number of tokens being transferred for ERC-20 tokens is in a different format.
3. For now we should just use the name and symbol functions for the ABI to query the blockchain.

Here is the source code for a standard ERC-721 token:

contract ERC721 {
   // ERC20 compatible functions
   function name() constant returns (string name);
   function symbol() constant returns (string symbol);
   function totalSupply() constant returns (uint256 totalSupply);
   function balanceOf(address _owner) constant returns (uint balance);
   // Functions that define ownership
   function ownerOf(uint256 _tokenId) constant returns (address owner);
   function approve(address _to, uint256 _tokenId);
   function takeOwnership(uint256 _tokenId);
   function transfer(address _to, uint256 _tokenId);
   function tokenOfOwnerByIndex(address _owner, uint256 _index) constant returns (uint tokenId);
   // Token metadata
   function tokenMetadata(uint256 _tokenId) constant returns (string infoUrl);
   // Events
   event Transfer(address indexed _from, address indexed _to, uint256 _tokenId);
   event Approval(address indexed _owner, address indexed _approved, uint256 _tokenId);
}

Here is the ABI I'm using for name and symbol for ERC-721 on the community explorer:

https://github.com/acravenho/contract-verification/blob/master/contracts/erc721.json

[alexgaribay]

@acravenho When it comes to indexing, I shouldn't parse the data field for what are supposed to be ERC-721 token transfers? For ERC-20, I'm parsing that field for the amount of tokens being transferred.

[acravenho]

@alexgaribay I believe the data field is an extra field that could be used to show an image or some other data. I'm looking for clarification on this and will keep digging.