cli: Support unicode exceptions on Python 2 properly.

This reverts the previous usage of

  from __future__ import unicode_literals

in favor of using the u"" string prefix where needed.

Forcing everything to be unicode literals on Python 2
was a bad idea since the standard library is largely
bytestrings and that doesn't mix well.
Drop support for Python 3.2.

This is because we need to use the u"" prefix which
is not available in Python 3.0-3.2 to support
unicode error messages from plugins.

We now only support Python 2.6, 2.7, 3.3 and 3.4.