Compatibility fixes.

* Use unicode_literals thoughout the package.
* Add indexes to '{}'.format(), unindexed version unsupported in Py2.6.
* Remove remnants of Python 2.5 support (Django 1.5 is required anyway).
* Stop relying on `hasattr` on OneToOneField, won't work on Python 3.2 + Django 1.6
  (see https://code.djangoproject.com/ticket/21563).