Adds audio station - new name: dsdl

• Downloads also audio playlists via AudioStation.
• CLI now contains commands
  • photo - works just like in 1.x
  • audio - works just like photo, execept for --playlist parameter

Migration from 1.x:

• Adapt to new binary name dsdl
• Add photo command

Example

• Used to be
  ./pstd --user your-diskstsation-user --output photos/ http://diskstation/photo
• Now is
  ./dsdl photo --user your-diskstsation-user --output photos/ http://diskstation/photo