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