Repository name is required when creating a repo

I was trying to create a repository and I wasn't sure which parameters
were required. Following our philosophy of exposing required parameters
in the constructor, I change the `NewRepository` object to take in a
repository name and to make that property readonly.

Octokit.Tests.Integration/Reactive/ObservableIssuesClientTests.cs

@@ -18,7 +18,7 @@ public class ObservableIssuesClientTests : IDisposable
 
         _client = new ObservableIssuesClient(github);
         _repoName = Helper.MakeNameWithTimestamp("public-repo");
-        var result = github.Repository.Create(new NewRepository { Name = _repoName }).Result;
+        var result = github.Repository.Create(new NewRepository(_repoName)).Result;
         _createdRepository = result;
     }