Changed __unicode__ to __str__

``get_short_description`` calls ``force_unicode`` which on Python 3.x uses the representation returned by ``__str__()``. TextPlugin did not previously define a __str__ override so it used the definition from the base CMSPlugin class, which just displays the PK. This meant in structure editing mode, a placeholder would unhelpfully show "Text 22" rather than a bit of truncated text to help distinguish instances.