[1.0.x] Fix #14, respect storage.prefix when looking for files

The function ``find_all_files`` ignored ``storage.prefix`` when looking
for files, which made it fail when a prefix was used in
``STATICFILES_DIRS``.

(cherry picked from commit 187a7a72bf72370c739f3675bef84532e524eaf1)
Release 1.0.3