Changed default exception management

For some reason, in some unknown settings, Operation.exception could not reliably determine its superclass via a call to super. In such a scenario the method would fall back to ComposableOperations::OperationError as return value even if the superclass did indeed specify a different default exception. Hence, Operation.raises has been changed to define a singleton method instead of just assigning the default exception to an instance variable.
Merges Bug Fix: Default Exception Inheritance
Updates version: v0.8.0 -> v0.8.1
Release v0.8.1