Enabling stable-coin payments on GreenSea

Adding the option to execute/accept payments in stable-coin (i.e. bridged DAI) will allow creators to sell their work at a fixed price, as well as providing an alternative to those buyers and sellers that don't want to be exposed to market price volatility. Stable-coin payments would apply a protocol fee of 2% (instead of 1% as it currently is for SUSU payments). The extra revenues will be used to buyback SUSU, then burn it, therefore reducing the circulating supply.