Move to official Reactive Extensions for Xamarin.*

Xamarin is starting to ship official versions of Rx so we don't have to build our own. In fact, building our own is causing problems because Xamarin strong names their binaries. Unfortunately because Xamarin doesn't build Microsoft.Reactive.Testing, we have to roll our own, built against the Xamarin SN'd binaries. This also updates all of the old ReactiveUI bits to current versions.

## Here be Dragons

This is probably going to Cause Some Pain™, because people will have to be on the very latest stuff (especially for Xamarin.Mac). If you're seeing reference errors, that's why. OTOH, we're already seeing these problems already, so there's not too much to say.