Added extended attribute 'where from' logging for downloads

On OS X it is possible to get list of extended attributes of a file. For some file (e.g. downloads) these lists contain *where from?* information indicating website that the file came from originally.
It would make sense to extract all plist handling to one function though.
Especially that I found out the other method we have for reading plists is [obsolete](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSPropertyListSerialization_Class/Reference/Reference.html#//apple_ref/occ/clm/NSPropertyListSerialization/propertyListFromData:mutabilityOption:format:errorDescription:) in favor of `NSPropertyListSerialization.propertyListWithData_options_format_error_`.
We may consider changing it during some refactoring.