Bosesoundtouchapi

* Removed `xmltodict` requirement.
* Added method `ContentItem.ToDictionary` to return a dictionary representation of the class.
* Added method `MediaItemContainer.ToDictionary` to return a dictionary representation of the class.
* Added method `NavigateItem.ToDictionary` to return a dictionary representation of the class.
* Updated `NavigateResponse.ToDictionary` method to remove the xmltodict requirement.
* Added method `Preset.ToDictionary` to return a dictionary representation of the class.
* Updated `PresetList.ToDictionary` method to remove the xmltodict requirement.
* Added method `Recent.ToDictionary` to return a dictionary representation of the class.
* Updated `RecentList.ToDictionary` method to remove the xmltodict requirement.
* Updated `SearchResponse.ToDictionary` method to remove the xmltodict requirement.
* Added method `SearchResult.ToDictionary` to return a dictionary representation of the class.
* Updated `SearchStationArtists.ToDictionary` method to remove the xmltodict requirement.
* Added method `SearchStationResults.ToDictionary` to return a dictionary representation of the class.
* Updated `SearchStationSongs.ToDictionary` method to remove the xmltodict requirement.
* Added method `SourceItem.ToDictionary` to return a dictionary representation of the class.
* Updated `SourceList.ToDictionary` method to remove the xmltodict requirement.

* Updated `NowPlayingStatus.ContainerArtUrl` property to return the correct image url of the playing content: the `ArtUrl` value is returned if present; otheriwse the `ContentItem.ContainerArt` url is returned if present; otherwise, None is returned. Prior to this fix, the `ContentItem.ContainerArt` value was considered first which did not always match the `ArtUrl` value and thus caused an incorrect image to be displayed for the currently playing artist and track value.

* Added `NowPlayingStatus.ContainerArtUrl` property to return the `ContentItem.ContainerArt` url if present; otherwise, the `ArtUrl` value is returned.

* Added method `SoundTouchClient.UpdateNowPlayingStatusForSource` to update source-specific NowPlayingStatus object for a given source and sourceAccount value.
* Updated model `NowPlayingStatus` to allow creating an instance with specified values.
* Replaced `requests` import with `urllib3.request` import in `SoundTouchFirmware` class.

* Updated `SoundTouchWebSocket`.`NotifyListeners` method to suppress logging of exception details to the system logger for websocket error events.

* Updated `SoundTouchWebSocket`.`StopNotification` method to ensure that the underlying event loop thread is shut down when notifications are stopped.
* Updated `SoundTouchWebSocket`.`StartNotification` method to ensure that the underlying event loop thread is a daemon thread, so it does not delay process termination.