Rename to Octokit to be consistent with other API libs

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

Octokit/Http/ICredentialStore.cs

@@ -0,0 +1,11 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Octokit.Http
+{
+    public interface ICredentialStore
+    {
+        [SuppressMessage("Microsoft.Design", "CA1024:UsePropertiesWhereAppropriate", 
+            Justification = "The credential store migth not be immediate")]
+        Credentials GetCredentials();
+    }
+}