Add 'since` parameter for public repositories

Octokit/Models/Request/PublicRepositoryRequest.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Octokit
+{
+    [DebuggerDisplay("{DebuggerDisplay,nq}")]
+    public class PublicRepositoryRequest : RequestParameters
+    {
+        public PublicRepositoryRequest()
+        {
+        }
+
+        public long Since { get; set; }
+
+        internal string DebuggerDisplay
+        {
+            get
+            {
+                return String.Format(CultureInfo.InvariantCulture, "Since: {0} ", Since);
+            }
+        }
+    }
+}