apps.core.models.period: Fixed bug in __unicode__.
apps.core.models.period: Fixed bug in __unicode__.

__unicode__ is not used anywhere that matters, however it is good to
have it correct in any case.

vegarang/devilry-django,devilry/devilry-django,devilry/devilry-django,vegarang/devilry-django,devilry/devilry-django,devilry/devilry-django