Without conflicting with #86.

This is almost entirely trivial changes:

- Make dummy methods that should never be called either raise
  NotImplementedError or call self.fail() in tests.
- Standardize the no-coverage pragma
- Add a few pragmas where coverage varies across
  versions/implementations, mostly for the sake of tox (travis should
  hit them all).

We exclude interfaces declared with the 'class' statement, because their
methods typically have only a doctstring in the suite, which confuses
the branch coverage machinery.

We *can't* test the classAdvice stuff under Py3k.  Need to deprecate it
before the 4.0 release.