Move to Portable Libraries

In ReactiveUI 5.0, support is being dropped for WP7.x and .NET 4.0, meaning that it makes more sense now to move to Portable Libraries.

## Reorganize All The Things

Now that certain types have moved around in .NET 4.5 (most notably, `ICommand` is now in mscorlib), this becomes a more compelling proposition. Mainly:

1. RxUI.Xaml now becomes more like RxUI.Cocoa or RxUI.Android - *only* providing platform-specific helpers.
1. All of the commanding stuff moves into RxUI.Core
1. ReactiveUI.Routing goes away, its stuff moves into RxUI.Core and RxUI.Xaml

## TODO:

- [x] - Move ReactiveUI Core to PLib
- [x] - Reorganize everything
- [x] - Fix MakeRelease.ps1 to build Portable packages