Deborgified the app cache.

Improved Andrew's hack to create temporary app caches to handle
migrations. Now the main app cache has a "master" flag set to True
(which is a non-default keyword argument, thus unlikely to be used by
mistake). Other app cache instances have "master" set to False.

The only sanctioned way to access the app cache is by importing
django.core.apps.app_cache.

If you were instanciating an app cache and relying on the Borg pattern,
you'll have to refactor your code.

tests/migrations/models.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from django.core.apps.cache import BaseAppCache
+from django.core.apps.cache import AppCache
 from django.db import models
 from django.utils.encoding import python_2_unicode_compatible
 
@@ -12,7 +12,7 @@ class UnicodeModel(models.Model):
 
     class Meta:
         # Disable auto loading of this model as we load it on our own
-        app_cache = BaseAppCache()
+        app_cache = AppCache()
         verbose_name = 'úñí©óðé µóðéø'
         verbose_name_plural = 'úñí©óðé µóðéøß'
 
@@ -32,4 +32,4 @@ class UnserializableModel(models.Model):
 
     class Meta:
         # Disable auto loading of this model as we load it on our own
-        app_cache = BaseAppCache()
+        app_cache = AppCache()