Switchedto PUT for setting playlists

This commit switches to the PUT HTTP method to set or replace playlists
at /playlist/<name>. In case the playlist already exists, it is
replaced.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Unifying data by introducing serialization method

This commit introduces a serialize method for all database objects
making it easier to get a unified representation of the same data.

Additionally some parts of the database were restructured. This hold
especially for the relationship between player and playlist with the
player object now holding the song currently playing.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Method to get expand settings

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Fix moving playlist items up/down

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Fix playlist entry move

Signed-off-by: Lars Kiesow <lkiesow@uos.de>