Add possibility to configure GitHubClient timeout (#963) (#1693)

* Add possibility to configure GitHubClient timeout (#963)

A first attempt to fix the problem describe in #963 by adding a possibility
to extend the default timeout value (100s)
that is too short to be able to post assets in github release.

* Rename to SetRequestTimeout
Make comments consistent

Octokit/IGitHubClient.cs

@@ -7,6 +7,16 @@ namespace Octokit
     /// </summary>
     public interface IGitHubClient : IApiInfoProvider
     {
+        /// <summary>
+        /// Set the GitHub Api request timeout.
+        /// Useful to set a specific timeout for lengthy operations, such as uploading release assets
+        /// </summary>
+        /// <remarks>
+        /// See more information here: https://technet.microsoft.com/library/system.net.http.httpclient.timeout(v=vs.110).aspx
+        /// </remarks>
+        /// <param name="timeout">The Timeout value</param>
+        void SetRequestTimeout(TimeSpan timeout);
+        
         /// <summary>
         /// Provides a client connection to make rest requests to HTTP endpoints.
         /// </summary>