Updated DropboxUploader class

Hi,

   Current version of DropboxUploader doesn't work due to changes in www.dropbox.com . You can follow the discussion here: https://github.com/jakajancar/DropboxUploader/issues/37 .Currently, the bug is not oficially fixed yet, but there is a pull request that works. I got this new code and update this repository. Now everything works again. This pull request update this class.

Cheers!