Add a configuration option to define the permissions of created file system directories

[andrehufdim]

When papaya creates the media and cache directory structue they are created readable, writeable and executeable for all (777, rwxrwxrwx).
When saving a media file it is created readable and writeable for all (666, rw-rw-rw-). Cache files are created readable and writeable for the owner and readable for group and others (644, rw-r--r--).

In some cases 600/700 should be enough, so only the owner (the user who runs the webserver; www-data per default on a debian/ubuntu server) can open the directories and read/write the files.