Support ERC-20, ERC-223 and ERC-721 tokens

To play nicely with the Ethereum token and asset economy the HTLC contract needs to support the various widely used token standards.

The core HTLC logic should be separated into it's own contract, which can then be used by frontend contracts for the specific token.

## TODO

- [ ] add `address token` to `Exchange` struct
- [ ] When `token == address(0x0)` it's Ether? Or separate contract for Ether.
- [ ] Write tests for ERC-20 token
- [ ] Python client support for the supported tokens
- [ ] Include token address in `HASH(...) = exchGUID`

Start by locating the HTLC contract and understanding how Exchange data and HASH(...) = exchGUID are currently defined. Review the existing contract tests, then plan separate Ether and token-facing contracts, ERC-20 tests, and Python client support for the supported standards. Done means the listed token standards work without breaking the core HTLC flow.

Written by the indexing model from the issue text.