Proposed fix for https://github.com/xunit/xamarin.xunit/issues/2

The issue was cause by the monotouch linker optimising away unreferenced assemblies, in this case xunit.runner.ios and xunit.runner.utility, and they were therefore not being bundled with the application.

In the appdelegate.cs example we **must** uncomment line 37:
// We need this to ensure the execution assembly is part of the app bundle
**runner.AddExecutionAssembly(typeof(ExtensibilityPointFactory).Assembly);**

And explicitly install the unit.runner.utility package:
**Install-Package xunit.runner.utility -Pre**

These two changes ensure that these two assemblies are bundled in with the compiled app, otherwise the linker will optimise them away as they are not directly referenced in the code.