Support 'skip' feature of newer unittest

This adds 'skip' feature to zope.testrunner, as proposed by newer version of unittest.

Basically, it just changes the rendering offered by zope.testrunner so that skipped tests are separated from the other tests.
It's a proposed fix for https://bugs.launchpad.net/zope.testrunner/+bug/880207 with a slightly different implementation though.