Prepare a Nuget package

Some changes in order to publish the core project as a NuGet package.

It is available at https://www.nuget.org/packages/FSharpVSPowerTools.Core/.
Not necessarily. The NuGet package will pick up the latest FCS upon downloading. But yes, we should modify it when FCS has breaking changes. We could move this to `nuspec` file if that is better.