Portable ReactiveUI.dll, even on Xamarin

This PR makes ReactiveUI.dll portable on all platforms (i.e. built via PCL Profile78), even on Xamarin. Here's what we did, since it's not super-obvious from the replaced binaries:

* We upgrade Rx to 2.2
* All of the Mono / iOS / Android binaries are now the **Microsoft** versions of the binaries, including the PCL versions. Since Xamarin.iOS / Android are basically standard .NET 4.5 (with caveats), we can use the `PlatformServices` DLL from the Net45 folder in the MS Rx binaries.
* ReactiveUI_* is now Dead, the Xamarin SLN file now includes ReactiveUI.csproj
* All of the platform DLLs now reference the PCL ReactiveUI.dll