Add support for getting (multiple) FileURLs

[tobywf]

This should fix issue #9 . I decided to go with a different interface, since it's possible to select multiple files. Reading public.file-url as a string does work, but I'm not sure if this is great. Who can say with Apple's documentation? There are multiple ways of getting the information

• stringForType works
• URLFromPasteboard, which NSPasteboardTypeURL mentions but NSPasteboardTypeFileURL doesn't, even though FileURLs are just NSURLs with isFileURL == YES
• readObjectsForClasses

What leads me to believe stringForType isn't great is that setting file URLs via setString:forType doesn't work great. It makes my Finder spin for quite a while. Instead of exposing FileURL for get_contents and maybe having a special case in set_contents, I think this approach is quite nice.