Look for unittest2 in skip plugin

If unittest2 is available, try to use its SkipTest class rather than a
nose internal version.  Inspired by better integration with testtools
for older python releases (see [1]).

[1] https://review.openstack.org/#/c/33056/
Merge pull request #699 from ianw/skiptest-exception

Look for unittest2 in skip plugin
Update changelog with Ian's contribution.