Package dot-net Centroid as a NuGet package

Added a new rake task, ```package:cs``` which creates the NuGet package for the .NET library. I consider creating the NuGet package / gem / etc. separate from publishing them, which isn't included here.

The current nuget spec has hardcoded values in the rake task for things like version, author, description, etc--we'll want to change that once we add the other languages' packages, but I wanted to start somewhere.

Also note that I made the package task depend on the tests--just a preference but I prefer to not package things that don't pass tests and I'd rather just let rake figure that out rather than running the two myself (e.g test then package).