Get user model and username field in a more lazy way

User model will not be ready at import time

Signed-off-by: Ilya Baryshev <baryshev@gmail.com>
Django 1.7: migrations

We're keeping old migrations in south_migrations folder to
be backwards-compatible with Django<1.7 and South>=1.0
Django 1.7: Explicit fixtures in tests

Using of initial_data in tests is problematic due to new app loading mechanism.
Because initial_data fixtures is deprecated in favor of migrations, initial_data
fixtures are renamed (to avoid confusion).
They're explicitly specified in testcases.
Django 1.7: Lazy loading of User model in management command

After app loading change, User model can no longer be referenced
while app registry is not populated.

This commit completes work in c7aa6c8
Django 1.7: Fix ApiAccess __unicode__ for py3
Django 1.7: Fix customuser tests

Due to app loading changes app_label is no longer effective,
so CustomUser model is redefined in tests.
Fixture name is changed to avoid collision with contrib.auth fixture
Django 1.7: Update tox config

Update Django and django-oauth-plus versions
Django 1.7: Minor testsuite fixes

* RuntimeError in related_resource, due to model name conflict
* ValueError when assigning to non-nullable field
* Add explicit MIDDLEWARE_CLASSES to avoid checks framework warnings
Django 1.7: Fix PutListNestResouceValidationTestCase

Attribute 'pk' is ignored, use 'id' instead.
Due to non-monotonic autoincrement in sqlite prior 1.7 test was passing:
both notes were deleted and created anew (pk were not taken into account,
they just "fortunately" matched pks of newley created models).
Django 1.7: Update tox to latest Django release
Merge remote-tracking branch 'coagulant/feature/django-1.7'