Changes default external file naming to use the UUID as the file name.
Original file name is saved in original_filename property.
Added :original_name option to DulHydra::FileManagement#add_file to provide
a means to bypass setting the original_filename.

The change was made because using original file names are unpredictable and may be
undesirable. Non-roman characters, spaces, etc. may cause problems; also in some
cases there could be a privacy or security concern in exposing the file name.