#22 Interface for AbstractUserException

[Tim-Oliver Paaschen]

Currently an exception must derive from AbstractUserException to be transmitted by IIOP.NET. This prevents the use of own base classes. By adding an (empty) interface IAbstracUserException this limitation can be avoided.

Derive the class AbstractUserException form the new interface, change the signature of GiopMessageBodySerialiser.SerialiseUserException() to take the interface as input parameter and change the type-checks to check for the interface instead of the class AbstractUserException.