This is a big one.
- Cleaned up XtendInterpreter and put helper methods in util class.
- Moved stacktrace functionality from TracingInt to XtendInt
- Revamped XtendInterpreter API; there is now an evaluateMethod() method
which should be used instead of the old evaluate(). A number of setter
and configure methods are not API any more.
- Said method also accepts an IClassManager, which therefore does not
have to be injected into the interpreter any more, which solves the
problem of needing a global singleton. 
- Established IObjectRepresentationStrategies, which abstract away the
representation of instances of Xtend classes. This will allow working
with instances of Xtend classes for which there is no compiled Java
class. This is only partly implemented so far.