Backup the application config files.

        Algorithm:
            if exists home/file
              if home/file is a real file
                if exists mackup/file
                  are you sure ?
                  if sure
                    rm mackup/file
                    mv home/file mackup/file
                    link mackup/file home/file
                else
                  mv home/file mackup/file
                  link mackup/file home/file

def backup(self):
        """
        Backup the application config files.

        Algorithm:
            if exists home/file
              if home/file is a real file
                if exists mackup/file
                  are you sure ?
                  if sure
                    rm mackup/file
                    mv home/file mackup/file
                    link mackup/file home/file
                else
                  mv home/file mackup/file
                  link mackup/file home/file
        """
        # For each file used by the application
        for filename in self.files:
            (home_filepath, mackup_filepath) = self.getFilepaths(filename)

            # If the file exists and is not already a link pointing to Mackup
            if ((os.path.isfile(home_filepath) or
                 os.path.isdir(home_filepath)) and
                not (os.path.islink(home_filepath) and
                     (os.path.isfile(mackup_filepath) or
                      os.path.isdir(mackup_filepath)) and
                     os.path.samefile(home_filepath,
                                      mackup_filepath))):

                if self.verbose:
                    print("Backing up\n  {}\n  to\n  {} ..."
                          .format(home_filepath, mackup_filepath))
                else:
                    print("Backing up {} ...".format(filename))

                if self.dry_run:
                    continue

                # Check if we already have a backup
                if os.path.exists(mackup_filepath):

                    # Name it right
                    if os.path.isfile(mackup_filepath):
                        file_type = 'file'
                    elif os.path.isdir(mackup_filepath):
                        file_type = 'folder'
                    elif os.path.islink(mackup_filepath):
                        file_type = 'link'
                    else:
                        raise ValueError("Unsupported file: {}"
                                         .format(mackup_filepath))

                    # Ask the user if he really want to replace it
                    if utils.confirm("A {} named {} already exists in the"
                                     " backup.\nAre you sure that you want to"
                                     " replace it ?"
                                     .format(file_type, mackup_filepath)):
                        # Delete the file in Mackup
                        utils.delete(mackup_filepath)
                        # Copy the file
                        utils.copy(home_filepath, mackup_filepath)
                        # Delete the file in the home
                        utils.delete(home_filepath)
                        # Link the backuped file to its original place
                        utils.link(mackup_filepath, home_filepath)
                else:
                    # Copy the file
                    utils.copy(home_filepath, mackup_filepath)
                    # Delete the file in the home
                    utils.delete(home_filepath)
                    # Link the backuped file to its original place
                    utils.link(mackup_filepath, home_filepath)
            elif self.verbose:
                if os.path.exists(home_filepath):
                    print("Doing nothing\n  {}\n  "
                          "is already backed up to\n  {}"
                          .format(home_filepath, mackup_filepath))
                elif os.path.islink(home_filepath):
                    print("Doing nothing\n  {}\n  "
                          "is a broken link, you might want to fix it."
                          .format(home_filepath))
                else:
                    print("Doing nothing\n  {}\n  does not exist"
                          .format(home_filepath))