WinRT support 

Hey mate,

This is probably still WIP at the moment, but I've managed to get something to build, and was able to get a simple request working from a dummy WinRT app. :metal: 

This is the first time I've even touched WinRT, so I had a few headaches and even managed to convince myself it didn't work for a while because _deadlocks_. :angry: 

Anyway, I think it's close, but I'm not really sure how to put it through more thorough testing. I'm not even sure where to begin with unit testing WinRT, so if you have any ideas I'd love some help.

The approach I took was just to polyfill all the reflection stuff and some static `Uri` methods that are absent (or moved) in WinRT. If that's the wrong approach, or you just have a better one, I'm open to suggestions.

Cheers
Probably also need to copy-paste the portable version as a WPA81 version