Rename to Octokit to be consistent with other API libs

GitHub is naming all of the libraries Octokit for their respective
platforms

Octokit.Reactive/IObservableGitHubClient.cs

@@ -0,0 +1,12 @@
+namespace Octokit.Reactive
+{
+    public interface IObservableGitHubClient
+    {
+        IObservableAuthorizationsClient Authorization { get; }
+        IObservableAutoCompleteClient AutoComplete { get; }
+        IObservableOrganizationsClient Organization { get; }
+        IObservableRepositoriesClient Repositories { get; }
+        IObservableSshKeysClient SshKey { get; }
+        IObservableUsersClient User { get; }
+    }
+}