NuGet all the things!

This PR fixes #582 by getting all the packages from NuGet. Locally I have build the RxUI packages except for the `mac` files to verify that everything works.

## What else?

This PR does a number of other things.

### Updates to the .nuspec files

The .nuspec files are updated to include the new PCL profiles and the new WPA81 folder.
* NuGet.exe has been updated to 2.8.2.
* RxUI.Blend now contains the Blend projects. Putting this in platforms does not make sense since it is only for some Xaml platforms.
* RxUI.Platforms now references RxUI.Core and RxUI.Blend.
* Remove the files for RxUI.Mobile.
* Make sure the .pdb files are tracked.
* Fix a bug in `MakeRelease` where a single empty file would not result in an error during the creation of the packages.
* Kill the `WinRT45` identifier and replace it with `Win8` for consistency.
* Fix the Winforms and AndroidSupport packages.

### Kill all nuget.targets.

Automatic package restore is way better, so let's use that.

### Add missing Testing project

The RxUI.Testing project for Windows 8 was missing and has been added.

### Bug fixes to RxUI.Events.

* The EventBuilder project now also uses packages from NuGet.
* Events for iOS failed to build with Xamarin 3.0 and possibly earlier.
* Events for Android failed to build and now references the assemblies in the `v1.0` folder.
* Events for WIndows 8 failed to build because it referenced thee wrong .winmd files.
* Really reference Android 4.0.3 files.

### Small fixes.

* Make sure all projects output a pdb and xml file in release mode.
* Fix some configuration mistakes and output folders.
* Missing link to CommonAssemblyInfo.
* Fix a reference to a .Net 4 assembly leading to `app.config` everywhere.

## TODO:

- [x] Only reference RxUI.Blend for the Xaml platforms that support it.
- [x] Decide if we would update RxUI.Mobile and reference RxUI.Core similar to RxUI-Xaml. It should.
- [x] Decide if RxUI.Core should have a dependency on Newtonsoft.Json. It shouldn't.
- [x] In `ext` is still `XamMac.dll` and `Xamarin.Android.Support.v13.dll`. The support lib is also in the reference assemblies folder. Should we use that one? What about `XamMac.dll`?