Patch for #45

As explained in #45, the pdb plugin must check for SkipTest and DeprecatedTest explicitly, as all other plugins have to. The fix until now was a workaround that prevented SkipTest from getting to *any* plugins. Another possibility to prevent pdb from triggering is to bring back addSkip and call it instead of addError.
I also removed test_issue_101 (it's #101 from Google Code), because that issue was caused by the same problem and it's not an issue any more.