Ship Akavache 3.0

This branch is the Master branch for tracking all of the stuff that will eventually become Akavache 3.0

# What's (Going To / Possibly Be) New?

## New Features
- [x] Portable Library support thanks to @onovotny (originally tracked in #44)

## Maintenance / Cleanup

- [x] Clean up the use of static properties, especially around service injection. (tracked in #49)
- [x] Document All Of The Things™

## Cut Features
- [ ] Cloud Synchronization (Idea written up in #48)

## TODO:

- [x] Wait for [ReactiveUI 5.0 to ship](https://github.com/reactiveui/ReactiveUI/pull/219)