#872 Use NuGet package restore to install development dependencies.

Removes all of `Moq` and `xunit` from `Lib/`, these are now installed by NuGet package restore. Also removes the `Lib/NuGet/` in favour of the more conventional `.nuget/' directory.

Package restore is set to happen automatically on build, but this is implemented slightly differently than the approach that @nulltoken suggests on #872. Rather than writing a manual `BeforeBuild` event it relies on the standard `<RestorePackages>true</RestorePackages>` tag in the `csproj` file.