fixes bug on CtStatementImpl.insertAfter

- ClassCastException was being thrown when trying to cast to
CtBlock. now the cast is made to CtStatementList.
- added test class spoon.test.ctCase.SwitchCaseTest and
resource test class spoon.test.ctCase.ClassWithSwitchExample
to test both insertAfter and insertBefore.
Merge pull request #15 from cc18/master

fixes bug on for insertAfter on statements in case blocks