Added basic tests for get_app_config[s].
Made unset_installed_apps reset the app registry state.

Previously the _apps/models_loaded flags could remain set to False if
set_installed_apps exited with an exception, which is going to happen as
soon as we add tests for invalid values of INSTALLED_APPS.
Made the AppConfig API marginally more consistent.

Eliminated the app_ prefix that was more confusing than useful.
Normalized exceptions raised by AppConfig.create.

It raises ImportError whenever an entry in INSTALLED_APPS points
(directly or indirectly via AppConfig.name) to a non-existing module
and ImproperlyConfigured in all other cases.

Catching ImportError and re-raising ImproperlyConfigured tends to make
circular imports more difficult to diagnose.
Added tests for invalid values of INSTALLED_APPS.