supports loading a referenced class from source classpath

the method CtTypeReferenceImpl.getActualClass was throwing an
Exception when trying to load a class reachable from the
source classpath.

the unused methods StandardEnvironment.getClasspath() and
StandardEnvironment.setClasspath() were recycled to achieve this.

to test this new feature the TypeReferenceTest class was added,
along with its resources in src/test/resources/reference-test/.

this fix probably needs refactoring.