Integrate with MvvmCross

I've created a `Autofac.Extras.MvvmCross` project and NuGet package.

The [original implementation](https://nbevans.wordpress.com/2014/02/17/xamarin-mvvmcross-with-autofac/) came from @nbevans, which I refactored slightly before adding the tests myself.

The magic all happens in `AutofacMvxIocProvider` which implements `IMvxIoCProvider` from MvvmCross and wraps an `IContainer` from Autofac.

The tests all pass in Xamarin Studio in OSX, but it definitely needs someone else to confirm it works across other configurations.

I wasn't able to run the full build, as running `xbuild default.proj` fails because [`default.proj#L106`](https://github.com/autofac/Autofac/blob/master/default.proj#L106) isn't Mono-friendly.

Feedback welcome!