Handle path_exists for links in a more accurate way

Currently, we are checking if a path exists in windows just checking for the file name, and if it doesn't exists, we add the .lnk extension at the end of the file and check again (to be consistent with the linux api where a link can has any or none extension), because windows requires the .lnk extension. The problem is that (in windows) if we just wanted to know if the folder "my-book" (without any extension) exists and it doesn't, but instead of that a shortcut named my-book.lnk exists, our function "path_exists" (in: u1-client/ubuntuone/platform/windows/os_helper.py > path_exists) is going to tell us that the folder exists when the result is (kinda) not real.

The main issue is that in Linux you can't have both a file or a dir and a link with the same name, and in windows you can (if you ignore extensions).