Remove broken test "derive_tag_names"

The test needs to be adapted for the new SubjectParser class.
Fixes issue #28
Remove broken test "find_submitter"

The refactoring in 855eaf353796f2f71f917272cec09f3da5f2310c requires
this test to be refactored.
Refactor test_mailbox.py to cleanup after itself
Use "nose" as testrunner