Rename to Octokit to be consistent with other API libs

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

Octokit/Http/InMemoryCredentialStore.cs

@@ -0,0 +1,21 @@
+using System.Threading.Tasks;
+
+namespace Octokit.Http
+{
+    public class InMemoryCredentialStore : ICredentialStore
+    {
+        readonly Credentials credentials;
+
+        public InMemoryCredentialStore(Credentials credentials)
+        {
+            Ensure.ArgumentNotNull(credentials, "credentials");
+
+            this.credentials = credentials;
+        }
+
+        public Credentials GetCredentials()
+        {
+            return credentials;
+        }
+    }
+}