Ship ReactiveUI 5.0

This branch is the Master branch for tracking all of the stuff that will eventually become ReactiveUI 5.0 

## What MUST HAPPEN to Ship
- [x] Clean up the collection interfaces and classes (tracked in #287, because we can't do it post 5.0)
- [x] Rewrite ReactiveAsyncCommand to be more like ReactiveCocoa's RACCommand (because we can't do it post 5.0, #288)
- [x] Document everything that's not documented at the moment (tracked in #316)
- [ ] Fix the Website because it's a train wreck

## What WOULD BE AWESOME to Ship
- [x] Readonly reactive collections (tracked in #267)
- [ ] Fix the XYZPlayground samples, people read them
- [ ] Finish the PowerShell integration with RxUIViewModelGenerator
- [ ] Move Xamarin.* to new unified profile that they're going to release soon (Dubious if they'll actually release them soon)

### Notes

* Preferably, don't commit to this branch directly unless it's something small, do it via a PR
* Maintenance / Cleanup is definitely **more important** than New Features right now, since none of the uncompleted New Features require breaking changes. We can always ship them later. Maintenance will involve breaking changes.
* **Don't merge this to master yet until ReactiveUI 5.0 ships**