Eventloop Implementation for Android/iOS/Xaml/Mac/WinRT

See: https://github.com/paulcbetts/splat/issues/77

First run at an implementation. I haven't edited the project files yet, but I can do so once the code is reviewed. The Android/iOS/Xaml implementation have all been tested. The WinRT code appears to compile but I don't have access to the platform to test. The Mac code is nearly identical to iOS but I don't have a license to test it with.


Yeah, I would rather have creating an EventLoop be an explicit thing because it makes it easier to shut them down later (how do I get a reference to a secondary EventLoop from my Main thread in my shutdown code?).

We could make an `EventLoop.FromMainLoop` method to get the main loop dispatcher