[WIP] Universal App support.

This PR adds Universal App support to RxUI 6.x. 

## Changes

* **Switch from Profile78 to Profile259**. Profile259 is the new portable profile that should be used for future development. It includes Universal App support. The difference between these profiles is that WCF support is removed from the latter. To switch to this profile I needed to untick Xamarin.iOS and Android, since Xamarin does not yet support this profile in the UI. Manual references work fine.
* **Change all `.WinRT` projects to include Windows Phone 8.1 support.**

## Some things that need to be done

- [x] Xamarin currently does not support Profile259. We have to wait for them to release this, but for now a manual reference just works fine. It seems the latest Xamarin for Visual Studio Alpha release has support for the new profiles.
- [x] Wait for an Rx.NET update. For now a manual reference of the winrt files works fine. https://github.com/Reactive-Extensions/Rx.NET/pull/4 and https://github.com/Reactive-Extensions/Rx.NET/pull/10
- [x] The Events project for WinRT needs to be split up into separate project for WP81 and Win81.
- [x] Splat needs to switch to Profile259 https://github.com/paulcbetts/splat/pull/55.
- [ ] Add new assemblies for Windows Phone 8.1 Silverlight.