added trade cancellation stubs to CexTrader, included in the Trader module and moved TradeContext to own file inside the Trader module
Fixed several minor details in CexTrader and Added stub methods to
TradeContext

- Added the cancel_orders method to CexTrader
- changed the get_trade_context
- FIxed unused err and mistyped error in iterate#rescue

- Added stub fetch method to TradeCOntext
created class ReadonlyTrader