Updated DropboxUploader

Hi, 

   Since a week ago the DropboxUploader class doesn't work. The issue is commented here (Dropbox Api has modifications):

https://github.com/jakajancar/DropboxUploader/issues/25

  Yesterday some commits were accepted and now everything works. I have updated this class like a copy of the original repository and I have checked it. Everything works. I propose you to update this class with this pull request.

Thanks.