Ship ReactiveUI 5.0

This branch is the Master branch for tracking all of the stuff that will eventually become ReactiveUI 5.0 

## Ideas / TODO:

- [ ] Move to Portable Libraries / .NET 4.5 only (tracked in #217)
- [ ] Remove the cruft and compatibility shims that have built up over time from old platforms
- [ ] Kill RxApp with :fire:, replace with a more granular solution

#reactiveui-5-0