Added support for NuGet symbols package, for use with SymbolSource.org

Now build script will create two nuget packages, one with libs only, and second with libs and sources
http://docs.nuget.org/docs/creating-packages/creating-and-publishing-a-symbol-package

Added packing all dlls,pdbs and sources by change in nuspec file
Also added acknowledgements.txt, How to Use.txt and license.txt to nuget package, same as in release zip