Create classifieds market

[alcueca]

Start by using a fixed price buy/sell market as for AB. Please write it from scratch instead of reusing their contract and name everything as for a craigslist classifieds board.

The mechanics are:

1. The market works with a currency token, like Issuance.sol.
2. Someone puts a buy or sell advert. There is no description or anything for the advert, just an address as a payload.
3. The payload address conforms for an ERC721 contract interface.
4. The buy part puts in escrow an amount of the currency token, for the sell part the ownership of the ERC721 payload is what is put in escrow.