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.


At a very high-level I tried to model the API after the existing platform native implementations, all of  which are thread local. Each thread is associated with a specific Dispatcher/NSRunLoop/Looper/etc., IEventLoop is then a thin wrapper on top of native implementation. 

Also the Stop method is not guaranteed to be thread-safe (specifically for NSRunLoop) therefore it seemed to make sense to force access to the thread local IEventLoop via a static Current getter.