Add the ability to setup triggers

[ChriD]

Add the ability to setup triggers (trigger manager is already there in kernel) e.G. /setupMediaRendererTrigger?Id=Küche&trigger=volume This will now always return the request when the volume on virtual renderer Küche was changed

[ChriD]

Maybe in combination with #40 ? (Long polling availability) e.g.: /getRendererValue?id=Küche&key=Volume&trigger=true