fixes bug when pretty-printing an invocation

- added DefaultPrettyPritnerTest class to check that an invocation
"abs(message.length())" was being printed as "java.lang.Math.abs(message.length())"
- the problem is related to the building model part. this happened when an individual model was being created (one model for a .java file instead of a project folder) and a dependency of that file was not accessible from spoon.