Add support for recognition and handling of Windows junctions.

Hi Martin,

When I gave riffle a try on my Windows 7 machine, it failed for folders like 'C:\Documents and Settings' or 'C:\Users\USER\Cookies' with 'Windows Error 5: Access denied'. This happens because these folders are junctions that are softlinked to other, real directories. I added support for them (using existing functionality from jaraco.windows) and thought it may be worth sharing. Feel free to decline in case you don't fancy it, had fun writing it anyway ;)

Cheers,
Christoph