Changed unique constraint.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Changed __unicode__, __str__ methods of models.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Changed using django-shortuuidfield.

 * django-shortuuidfield has a bug in Python3, but not yet fixed.
   https://github.com/nebstrebor/django-shortuuidfield/pull/5
 * Then, use my pull-request until this issues will fix.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>
Added unit test of address model.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>