Add public order support (Swap)

[dmosites]

Relates to @airswap/swap Swap contract in https://github.com/airswap/airswap-protocols/tree/develop/source/swap.

If an order is hashed with a senderWallet value of null (0x000...) the Swap contract should allow the order to be filled by the msg.sender as the senderWallet address.

This could be implemented in the swap function (used by primary web app), which takes recipient as an argument. If that is null, the following line would transfer to msg.sender instead of recipient. https://github.com/airswap/airswap-protocols/blob/develop/source/swap/contracts/Swap.sol#L156