Fix setUpClass upcalls on Python 2.6.

``testtools.TestCase`` now inherits from unittest2.TestCase, which
provides a ``setUpClass`` for upcalls on Python 2.6.
(Robert Collins, #1393283)

Change-Id: Id56212e3d7d519c7b73d2e19d3e34013fac34544
Release 1.4.0

Change-Id: Ic9982654deea070aeeffc0a7e361271fe9927b69
Show import error details.

When an import error happens ``testtools.run`` will now show the full
error rather than just the name of the module that failed to import.
(Robert Collins)

Change-Id: I2d9d7a4659b51a5e3ea8000301ecb799ed30f463