Add ObservableUserGpgKeysClient and fixes to satisfy convention tests

Octokit.Reactive/Clients/ObservableUsersClient.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using System.Reactive.Threading.Tasks;
 
 namespace Octokit.Reactive
@@ -16,6 +17,7 @@ namespace Octokit.Reactive
             Followers = new ObservableFollowersClient(client);
             Email = new ObservableUserEmailsClient(client);
             Keys = new ObservableUserKeysClient(client);
+            GpgKeys = new ObservableUserGpgKeysClient(client);
             Administration = new ObservableUserAdministrationClient(client);
         }
 
@@ -77,6 +79,15 @@ namespace Octokit.Reactive
         ///</remarks>
         public IObservableUserKeysClient Keys { get; private set; }
 
+        /// <summary>
+        /// A client for GitHub's UserUser GPG Keys API.
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://developer.github.com/v3/users/gpg_keys/">User GPG Keys documentation</a> for more information.
+        /// </remarks>
+        public IObservableUserGpgKeysClient GpgKeys { get; private set; }
+
+
         /// <summary>
         /// A client for GitHub's User Administration API
         /// </summary>