Move storage class to subpackage, add PRIVATE_STORAGE_CLASS setting

The default `private_storage.storage.PrivateStorage` class now points to
the class that was imported through the new setting, theirby preserving
backwards compatibility.

private_storage/appconfig.py

 from django.conf import settings
 
+PRIVATE_STORAGE_CLASS = getattr(settings, 'PRIVATE_STORAGE_CLASS', 'private_storage.storage.files.PrivateFileSystemStorage')
 PRIVATE_STORAGE_ROOT = getattr(settings, 'PRIVATE_STORAGE_ROOT', None)
 PRIVATE_STORAGE_SERVER = getattr(settings, 'PRIVATE_STORAGE_SERVER', 'django')
 PRIVATE_STORAGE_AUTH_FUNCTION = getattr(settings, 'PRIVATE_STORAGE_AUTH_FUNCTION', 'private_storage.permissions.allow_superuser')

private_storage/storage/__init__.py

+"""
+Django Storage interface
+"""
+from django.utils.module_loading import import_string
+
+from private_storage import appconfig
+
+__all__ = (
+    'private_storage',
+    'PrivateStorage',
+)
+
+# Fetch the storage class
+PrivateStorage = import_string(appconfig.PRIVATE_STORAGE_CLASS)
+
+# Singleton instance.
+private_storage = PrivateStorage()

private_storage/storage.py → private_storage/storage/files.py

 """
-Django Storage interface
+Django Storage interface, using the file system backend.
 """
 from django.core.files.storage import FileSystemStorage
 from django.core.urlresolvers import reverse_lazy
 from django.utils.encoding import force_text
 
-from . import appconfig
+from private_storage import appconfig
 
-__all__ = (
-    'private_storage',
-    'PrivateStorage',
-)
 
-
-class PrivateStorage(FileSystemStorage):
+class PrivateFileSystemStorage(FileSystemStorage):
     """
     Interface to the Django storage system,
     storing the files in a private folder.
@@ -22,7 +17,7 @@ class PrivateStorage(FileSystemStorage):
         if location is None:
             location = appconfig.PRIVATE_STORAGE_ROOT
 
-        super(PrivateStorage, self).__init__(
+        super(PrivateFileSystemStorage, self).__init__(
             location=location,
             base_url=base_url,
             **kwargs
@@ -36,8 +31,4 @@ class PrivateStorage(FileSystemStorage):
     def url(self, name):
         # Make sure reverse_lazy() is evaluated, as Python 3 won't do this here.
         self.base_url = force_text(self.base_url)
-        return super(PrivateStorage, self).url(name)
-
-
-# Singleton instance.
-private_storage = PrivateStorage()
+        return super(PrivateFileSystemStorage, self).url(name)