Support ASP.NET vNext

This is the first part of #170

/cc @lodejard @davidfowl @Eilon @glennc
Not sure if it matters (presumably not), but I usually see this as `System.Xml` and not `System.XML`. So maybe it's more of an ugliness thing than a correctness thing. But I know that in vNext there's a lot more case sensitivity than we're used to.