Thanks to cdestigter for the report.

Also renamed app_cache to apps and "app cache" to "app registry".

Deprecated AppCache.app_cache_ready() in favor of Apps.ready().

Reverted 4a56a93c.

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.

This commit doesn't contain any code changes; it's purely a refactoring.

This commit also lays the groundwork for future tests for the
makemigrations command.

Signed-off-by: Jannis Leidel <jannis@leidel.info>

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3 bcc190cf-cafb-0310-a4f2-bffc1f526a37