added the playlist management service and moved some of the logic from playback view model to it. Also removed/added some event aggregator messages to better handle the playback workflow

added the ability to save a playlist by serializing the items in it
added the ability to load a saved playlist
Merge branch 'saveLoadPlaylist#63'

Conflicts:
	Client/Playback/PlaybackViewModel.cs
fixed some merge issues
moved the order of the controls on the playback audio view to mimic that of the bottom bar
modified the default bottom bar view so that it will always show the playlist button
added a large cover art placeholder image and fixed a layout issue with the audio playback view
fixed an issue where the audio/video partial view of the playback view would not show sometimes (probably bug in caliburn)