Created NuGet nuspec packages and added them to the build

I created NuGet nuspec packages for NUnit and NUnit.Runners. I think we talked about not including runners, but I thought it was necessary to include the runners so that they are available on continuous integration servers like TFS.

I used the id `NUnit3` so that they won't conflict with existing NUnit NuGet packages.

The NuGet packages can be built using the command,

```
build /p:Configuration=Release /t:DumpSettings;PackageNuGetAll
```

This will build and test all required projects and create the packages in the bin\Release folders. There are also targets for `PackageNuGetFramework` and `PackageNuGetRunners`.

The version is currently set to `3.0.0-alpha` so that it will only appear when users are looking at prereleases. The version is set using the `PackageVersion` and `PackageModifier` at the top of `NUnit.proj`.

I tested by setting up a local NuGet repository and using the packages in test projects targeting multiple .NET frameworks. The BCL assemblies are only dependencies if you are targeting .NET 4.0.

My only question is, should nunit.framework.dll be included with nunit-console.exe in the runners package? I assumed it should because it is referenced, but it runs fine without.