Add model extensions for school. Closes #85

aleksis/apps/ticdesk_org/model_extensions.py

+from jsonstore import CharField
+
+from aleksis.core.models import Person
+
+Person.field(school=CharField(verbose_name=_("Name of school")))
+Person.field(school_class=CharField(verbose_name=_("School class")))
+Person.field(school_place=CharField(verbose_name=_("Place of the school")))