:fire: ReactiveUI.Packages

This PR converts ReactiveUI to a [Bait and Switch PCL](http://log.paulbetts.org/the-bait-and-switch-pcl-trick). This means that there is now solely one giant ReactiveUI.dll, no more ReactiveUI.Xaml.dll et al.

This approach has a number of benefits:

* This largely fixes the "iOS Registration Bug" that trolls basically everyone on Xamarin.iOS sees, because now ReactiveUI.dll doesn't attempt to early-load ReactiveUI.Cocoa.dll which doesn't work
* Fixes RxUI being incompatible with Mono Linking by removing Fody and going back to the static constructor trick. I don't like it either :( 
* Less assemblies, less trickery

Fixes #575