Renamed AppCache to Apps.

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

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

tests/apps/models.py

@@ -0,0 +1,17 @@
+from django.apps.registry import Apps
+from django.db import models
+
+# We're testing app registry presence on load, so this is handy.
+
+new_apps = Apps()
+
+
+class TotallyNormal(models.Model):
+    name = models.CharField(max_length=255)
+
+
+class SoAlternative(models.Model):
+    name = models.CharField(max_length=255)
+
+    class Meta:
+        apps = new_apps