use async to iterate over async results list

Signed-off-by: Tomas Neme <lacrymology@gmail.com>
add optional callback to readPlaylists method

Signed-off-by: Tomas Neme <lacrymology@gmail.com>
process all items together, async.map them into Playlists and return the result

Signed-off-by: Tomas Neme <lacrymology@gmail.com>
pass error to callback if there is one

Signed-off-by: Tomas Neme <lacrymology@gmail.com>
createPlaylist returns an error parameter, drop it if present

Signed-off-by: Tomas Neme <lacrymology@gmail.com>
comments

Signed-off-by: Tomas Neme <lacrymology@gmail.com>