Universal app support.

This PR adds universal app support to ReactiveApp similar to https://github.com/reactiveui/ReactiveUI/pull/552.

## Changes

* **Switch from Profile78 to Profile259**. Profile259 is the new portable profile that should be used for future development (http://www.twitter.com/dsplaisted/status/451487953483620352). 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.
* **Rename all `.Win81` projects to `WinRT` and include Windows Phone 8.1 support.**

## Some things that need to be done

- [ ] Xamarin currently does not support Profile259. We have to wait for them to release this, but for now a manual reference just works fine or we can do some xml hacks.
- [ ] ReactiveUI needs to add Universal App support. For now manual references work. https://github.com/reactiveui/ReactiveUI/pull/552
- [ ] 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
- [ ] Splat needs to switch to Profile259 https://github.com/paulcbetts/splat/pull/55.