Support for Sound property for toasts (GDR3+)

I added support for the Sound property. This can further be extended to add support for a silent notification:
<wp:Sound Silent="true" />
as per
http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj662938(v=vs.105).aspx#BKMK_gdr3
but I leave that for Jeff to decide how to add properties on the xml entities in his code.