Simplify Bitstamp's cancel order interface

This now uses rescu's built-in exception handling functionality and simplifies XChanges' code significantly. 

Note that this introduces two changes that break XChanges' public API so version should probably be increased:
- `BitstampAuthenticated#cancelOrder` now returns `boolean` instead of the now redundant `BitstampBooleanResponse`
- `BitstampTradeServiceRaw#cancelBitstampOrder` now throws BitstampException instead of ExchangeException on failed attempt.