Organize Visual Studio solutions and integrate NuGet packaging.

There is now a [published NuGet package](https://www.nuget.org/packages/libzmq_v120/4.1.0) that contains **static/ltcg** and **dynamic** libs/dlls, **x86/x64**, **debug/release**, **symbols** and **includes**. VS solutions have been isolated to prevent output conflicts. A GSL template is used to generate the .nuspec, .targets and .xml (VS UI extension). The [czmq](https://github.com/zeromq/czmq) VS2013 projects are updated to pull in the NuGet package by default.