Adds missing Django 1.7 migration
Changed __unicode__ to __str__

Addresses same issue as https://github.com/divio/djangocms-text-ckeditor/pull/175
Merge pull request #22 from briandant/master

Adds missing Django 1.7 migration
Merge pull request #25 from GreenLightGo/patch-1

Changed __unicode__ to __str__
Updated form validation to enforce at least one link type chosen

Returns `cleaned_data` to be compatible with Django 1.4 & 1.6 (1.7 does not require it).