Upgrade to RxUI 6 and Akavache 4
Observe on the main thread
Implement the NLog logger
Improve the logging
Update Akavache and Lager
Merge branch 'master' into upgrade-rxui-akavache

Conflicts:
	Espera/Espera.Core.Tests/packages.config
	Espera/Espera.Core/packages.config
	Espera/Espera.View.Tests/packages.config
	Espera/Espera.View/packages.config
Merge was borked
Still borked
Update RxUI
Connect the artwork property immediately to its source
Merge branch 'master' into upgrade-rxui-akavache

Conflicts:
	Espera/Espera.Core/packages.config
	Espera/Espera.View.Tests/packages.config
	Espera/Espera.View/ViewModels/DirectYoutubeViewModel.cs
	Espera/Espera.View/ViewModels/ISongSourceViewModel.cs
	Espera/Espera.View/ViewModels/SongSourceViewModel.cs
	Espera/Espera.View/ViewModels/YoutubeSongViewModel.cs
	Espera/Espera.View/ViewModels/YoutubeViewModel.cs
	Espera/Espera.View/packages.config
Fix the merge
Update some of the dependencies
Yo, I heard you like RxUI updates
First pass on the command upgrades
Finished the ReactiveCommand migration
Merge branch 'master' into upgrade-rxui-akavache

Conflicts:
	Espera/Espera.View/ViewModels/NetworkSongViewModel.cs
Use the deprecated Akavache backend for now
Create a new in-memory cache for the settings if we are in a unit test runner
Fix the unit tests