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.


FWIW, the Thread Local only creates an instance of the object, but doesn't run the loop. The loop is only run from within Spawn by calling the platform native call for looping which would return you a reference to the event loop that you can use to shutdown. 

The nice part about using the thread local is that it gives us a convenient way to wrap the native primitive regardless of whether the loop was started using the IEventLoop API or the platform APIs directly.

I have another PR on the way that might help illustrate this better.