Add support for custom user model for Django 1.5.
Common compatibility wrapper for get_user_model().
Account for custom user models in migrations.

Note: Broken in South 0.8.3, fixed in 0.8.4.

When running migrations with 0.8.3 we recommend
upgrading to 0.8.4.

Implementation references:

  * https://gist.github.com/benjaoming/5605160
  * http://kevindias.com/writing/django-custom-user-models-south-and-reusable-apps/
  * http://stackoverflow.com/questions/15472704/trouble-migrating-reusable-django-app-models-to-use-a-custom-user-model

Related South bug:

  * http://south.aeracode.org/ticket/1320 (fixed in 0.8.4)