Makes AppendTextUnquoted public.

The Microsoft version is public, so should this be.
NuGet.MSBuild.Tools.CommandLineBuilderExtensions:AppendTextUnquotedIfNotNullOrEmpty attempts to call this method in NuGet for MSBuild NuGet package.
Backwards accuracy.

The method is only public in .NET 4+, previously it was protected.  Wrapping the access modifier in a #if NET_4_0 ensures that when building .NET 2, it is protected but when building 4+ it is public.
Merge pull request #1154 from Zoltu/patch-1

Makes AppendTextUnquoted public.