Moved volume conversion to utility class
Added "play playlist" action in service

Maybe not the best solution to handle playlist logic in the service, but
difficult elsewhere because of the need of a \"on complete\" callback,
which is easily accessed in the Service.
Implemented playlist driver using AudioService's playlist functionality

Perhaps would more appropriate with more playlist logic in driver than service,
see previous commit's message for additional reasoning.
Added missing license notice
Javadoc additions and naming updates
Handle the possibility that playlist is empty
Minor change to last commit
Merge branch 'playlist-driver' into development