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.
Normalized Model._meta.installed.

Used the information from the app cache instead of creating a duplicate
based on INSTALLED_APPS.

Model._meta.installed is no longer writable. It was a rather sketchy way
to alter private internals anyway.
Removed forgotten reference to the borg pattern.