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.


I'd actually like to ditch the SyncContext here and write something more like:

```cs
public interface IEventLoop : IDisposable
{
    Task PostAsync(Action block);
}
```

This way we can layer a generic `SynchronizationContext` implementation as an extension method:

```cs
public SynchronizationContext AsSyncContext(this IEventLoop This) {
    return new EventLoopSyncContext(This);
}
```

`EventLoopSyncContext` should be pretty easy to implement via the above `IEventLoop`.