Enable building with VS2010/.NET40 and fix NuGet restore

[All issues resolved, see last paragraph below.]

There were a number of package dependency problems and it turned out the main culprit was Newtonsoft.Json which needed to be version 4.0.8 for compatibility with RestSharp 102.7. After fixing that all the version problems went away and I was able to eliminate the package directory entirely and to rely on NuGet to download the packages at first build time.

The remaining dependency problem now is that the copy of Reactive.Xaml.dll that downloads from NuGet (3.0.1) is actually different from the version that was originally in the packages directory, so I copied it aside into packages-mod, and referenced just this single dll from there. Using the NuGet version fails. :-(

All these changes allowed reverting back to .NET 4.0 and the solution even builds and passes all tests in VS2010!

However, the VS2010 box needs to have .NET 4.5 beta installed, otherwise the build fails with "Warning	3	Unknown build error, 'Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.' 	ReactiveUI.Sample".

This relates to the fact that the reactiveui-* packages downloaded from NuGet, even when targetting 4.0 are being built on a machine that has 4.5 installed. Apparently this causes the problem above, so the reactiveui-* packages can't be used for building purposes on 'pure' 4.0 machines where 4.5 hasn't been installed. See http://connect.microsoft.com/VisualStudio/feedback/details/726702/moving-system-runtime-compilerservices-extensionattribute-to-mscorlib-breaks-structuremap-scans-of-namespaces-containing-extensionmethods-and-possibly-other-ioc-scenarios and a similar problem worked around here https://github.com/Code52/pretzel/pull/89 .

Final solution was to switch to using ReactiveUI 3.2.0 and Rx_Experimental. Now everything builds on VS2010/.NET40 even on machines without .NET45 installed.