Upgrade solution to VisualStudio 2015

The solution files should be upgraded to VisualStudio 2015. At the same time, now projects will be created to target specific .NET versions. The existing projects will be renamed to XXX-net2.0.csproj or XXX-net3.5.csproj, as appropriate. For example, the vs2008-nms.csproj will be renamed to vs2015-nms-net2.0.csproj. It will then be duplicated to vs2015-nms-net4.0.csproj to target the .NET 4.0 framework. Other platform specific versions can be created on an as-needed basis. These individual project files will all be included in the single vs2015-nms.sln file.