Add nuspec file for Nuget.

Hey, we're using this library at my job and I created a Nuget package out of MightyLittleGeodesy for our internal library server. This pull request is for the Nuspec file in case you would like to upload it to Nuget.

Having this file doesn't do any harm if you're not using Nuget. The project file or the code hasn't been changed at all.