Changed skipping of tests so it's not done in testcase.setUp but in plugins.

Reason: programmers don't expect that they must call super of setUp in their test classes (as no other testing tool requires it, including nose itself). Also code was a bit strange (setting TestCase.skipped in plugins so it can be skipped later in TestCase itself)

WARNING: You must remove call of super of setUp() in you test cases directly inherited from sane test cases as it was removed!

To be able to do this change, some deinitializations were moved from stopTest to afterTests, because when SkipTest is risen from startTest then stopTest is not called.