Xamarin support

This is a PR to include Xamarin support for xUnit.

## What's been done so far:
- [x] xunit.execution for iOS and Android
- [x] xunit.runner.utility for iOS and Android
- [x] Platform runner app for iOS 
- [x] Update Nuget packages
- [x] Platform runner app for Android

## What's left to do:
TEST!

Note that running all of xUnit's tests on Xamarin is not currently possible due to NSubstitute not being available there.

Smoke tests have been run to ensure that tests, theories and the like are properly detected and run.

### Limitations
Only Xunit 2 is supported. Xunit 1 appears to rely on an interface from System.Web and that assembly is not on Xamarin.

## Status
- iOS version works and is ready for wider testing.
- Android version works and is ready for wider testing.
- NuGet package interactions need to be validated