Skipped tests and subtest support

It is finally time to implement skipped tests. Issues #8 and #18 both implement test skipping, but they all failed some tests. With the refactoring of the test framework in #25, it was easy to add version-specific conditions to the gold files.  

This work is based on issue #18, with working tests on Py2.6, and also adds subtest support. Note that this issue creates conflicts with the gold file in #26, but I fixed these in my own master branch.