replace powershell install scripts with nuget 2.5 msbuild support

Replace powershell install scripts with nuget 2.5 msbuild support. This is the way to go now that it is build in and now that nuget 2.7 supports package restore. It is much simpler. No files get added to the solution. I octopack.targets simply gets added to the .csproj.

I also update the OctoPack solution itself to use nuget 2.7 package restore.