Bosesoundtouchapi

* Added Recently Played List cache processing. This allows a cache of recently played content items to be stored on the local file system. WebSocket support must be enabled for the caching to work, as it utilizes the nowPlayingUpdated event to drive played content to the cache.
* Updated `Recent` model with setter properties so that a recent item could be created without xml.

* 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.