Get All repo commits with request parameters

Implemented Get All using the available parameters wrapped in a
CommitRequest object.  Added unit tests and xml documentation along the
way.

Octokit/Models/Request/CommitRequest.cs

@@ -0,0 +1,47 @@
+using System;
+using System.Diagnostics;
+using System.Globalization;
+
+namespace Octokit
+{
+    [DebuggerDisplay("{DebuggerDisplay,nq}")]
+    public class CommitRequest : RequestParameters
+    {
+        public CommitRequest()
+        {
+        }
+
+        /// <summary>
+        /// SHA or branch to start listing commits from.
+        /// </summary>
+        public string Sha { get; set; }
+
+        /// <summary>
+        /// Only commits containing this file path will be returned.
+        /// </summary>
+        public string Path { get; set; }
+
+        /// <summary>
+        /// GitHub login or email address by which to filter by commit author.
+        /// </summary>
+        public string Author { get; set; }
+
+        /// <summary>
+        /// Only commits after this date will be returned.
+        /// </summary>
+        public DateTimeOffset? Since { get; set; }
+
+        /// <summary>
+        /// Only commits before this date will be returned.
+        /// </summary>
+        public DateTimeOffset? Until { get; set; }
+
+        internal string DebuggerDisplay
+        {
+            get
+            {
+                return String.Format(CultureInfo.InvariantCulture, "Sha: {0} ", Sha);
+            }
+        }
+    }
+}