Xamarin support

This is a PR to include Xamarin support for xUnit.

There are still many things left To Do, but I wanted to tee this up as a collaboration branch.

What's been done so far:
- Get Xamarin.iOS and Xamarin.Android support for the following libs: xunit.runner.utility, xunit.execution

What's left to do:
[] Platform runner app for iOS 
[] Platform runner app for Android
[] Run unit tests for xUnit on platforms
[] Update Nuget packages