Fix Bug 1073555 - Show an unsupported message to Windows RT users

I have learned that from https://miketaylr.com/posts/2013/10/navigator-dot-cpu.html

But... looks like the UA string also has `ARM` according to [MSDN](http://msdn.microsoft.com/en-us/library/ie/hh920767%28v=vs.85%29.aspx). I'll update the PR shortly to use it so tests can be added easily.