REFACTORING: removed addSkipBackupAttributeToItemAtURL: calls and NSFileManager category

it was used to exclude items saved for offline queue from backup. It is not needed, because offline queue is in CACHE directory, which is not included in backups by default.
REFACTORING: improved SHKItem saving + SHKOfflineSharer uses modern objective-c syntax
REFACTORING: SHKFacebook now uses ShareKit's new builtin routines for saving/restoring the item