Add a bunch of doc comments

We get a lot of build output because of missing XML comments that we
ignore. I'd like to stop ignoring them. To do that, we need to doc the
💩 out of everything.

Octokit/Models/Request/NewReference.cs

@@ -5,11 +5,23 @@ using System.Linq;
 
 namespace Octokit
 {
+    /// <summary>
+    /// Used to create a new Git reference.
+    /// </summary>
+    /// <remarks>API: https://developer.github.com/v3/git/refs/#create-a-reference</remarks>
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class NewReference
     {
         const string _refsPrefix = "refs";
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="NewReference"/> class.
+        /// </summary>
+        /// <param name="reference">
+        /// The name of the fully qualified reference (ie: refs/heads/master). If it doesn’t start with ‘refs’ and
+        ///  have at least two slashes, it will be rejected.
+        /// </param>
+        /// <param name="sha">The SHA1 value to set this reference to</param>
         public NewReference(string reference, string sha)
         {
             Ensure.ArgumentNotNullOrEmptyString(reference, "ref");
@@ -19,7 +31,21 @@ namespace Octokit
             Sha = sha;
         }
 
+        /// <summary>
+        /// The name of the fully qualified reference (ie: refs/heads/master). If it doesn’t start with ‘refs’ and
+        /// have at least two slashes, it will be rejected.
+        /// </summary>
+        /// <value>
+        /// The reference.
+        /// </value>
         public string Ref { get; private set; }
+
+        /// <summary>
+        /// The SHA1 value to set this reference to
+        /// </summary>
+        /// <value>
+        /// The sha.
+        /// </value>
         public string Sha { get; private set; }
 
         static string GetReference(string reference)