* Get as close to 100% coverage as possible.

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).

* Convert tox to run the coverage command, like travis. A follow up should have it produce coverage for all python versions/implementations.

Fixes #87

* Adjust coverage config for py2 only tests; fix typo; finish replacing self with cls

* Test sanity cleanups

Partially addresses #87

- Remove manual lists of tests in favor of loadTestsFromName. This
  mostly gets all test runners running the same number of tests.
- Remove `additional_tests`, which was just duplicating the discovered
  tests. No modern test runner seems to need it.
- Change the two compatibility skip decorators to actually use
  unitetest skips, which helps with the reporting.

* Per @tseaver, drop most test_suite functions.

* Port remaining doctest to unittest.

* Make the odd tests work on all supported python versions

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