Add Connection property to IObservableGitHubClient

Sometimes we need to be able to make raw requests. This
makes that easier.

Octokit.Reactive/IObservableGitHubClient.cs

@@ -1,7 +1,11 @@
-namespace Octokit.Reactive
+using Octokit.Http;
+
+namespace Octokit.Reactive
 {
     public interface IObservableGitHubClient
     {
+        IConnection Connection { get; }
+
         IObservableAuthorizationsClient Authorization { get; }
         IObservableMiscellaneousClient Miscellaneous { get; }
         IObservableOrganizationsClient Organization { get; }