Remove unneeded files for .NET Core (#1549)

* add .ruleset back to both Octokit and Octokit.Reactive

* rename Octokit.Core.sln to Octokit.sln

* remove platform-specific solution files

* remove .nuspec files

NuGet packaging information is now in the relevant project.json files

* remove ext folder containing platform-specific assemblies

* update the description of the .ruleset file we use

Octokit.Reactive/project.json

@@ -13,7 +13,10 @@
         "../Octokit/Helpers/Ensure.cs",
         "../Octokit/Helpers/Pagination.cs"
       ]
-    }
+    },
+    "additionalArguments": [
+      "/ruleset:../Octokit.ruleset"
+    ]
   },
   "packOptions": {
     "summary": "An IObservable based GitHub API client library for .NET using Reactive Extensions"