changed the error that occurs when no @TransformationInterface is
implemented by a @Transformation to a warning
Update ModelValidationHelper.java

adapted to allow the subtyping of transformations
Update ModelValidationHelper.java

adapted to allow the subtyping of transformations
Update XtendJavaValidator.java
Update README.txt
Merge pull request #3 from dwerle/master

Added possibility of creating subtypes of transformations