Add trade facility

In this commit, I’ve authored a bunch of classes that enable using the
trade system. Among them are:
AcceptTradeRequest - Accept a trade
CancelTradeRequest - Cancel a trade offer or deal
DeclineTradeOfferRequest - Decline a trade offer that was sent to you
DeclineTradeResponseRequest - Decline somebody’s response to your offer
GetPendingTradesRequest - Get a list of the trade offers and deals in
your pending trade box
ProposeTradeRequest - Propose a trade to somebody
ResponseToTradeRequest - Respond to a trade offer that was sent to you

This is my first time contributing - please feel free to help clean up my code!