Added a pre-receive environments client (#1796)

* Added pre-receive environment client

https://developer.github.com/v3/enterprise-admin/pre_receive_environments

* Added unit and integration tests for the pre-receive environments client

* moved test setup outside of tests

* fixed unit test

* Added reactive components for pre-receive environment endpoints

* Debugger display's and non public setters on response object conventions

* removed redundant parameter attributes

implemented update request independently of new request
changed ints to longs for future proofing
updated unit tests
changed update integration tests to create a new update request to ensure optional values function

* updating reactive environmentids to longs as well

* also converting response id to a long

* Fixed an incorrect unit test

renamed some test variable names for consistency
Added mockable ctors on responses, and parameterless ctors to maintain functionality

* Adding missing state parameter

* rename client member accessor to singular form in accordance with octokit naming conventions

* fixup tests

* add integration tests for observable client

Octokit/Models/Request/Enterprise/UpdatePreReceiveEnvironment.cs

@@ -0,0 +1,27 @@
+using System.Diagnostics;
+using System.Globalization;
+
+namespace Octokit
+{
+    /// <summary>
+    /// Describes an update to an existing pre-receive environment.
+    /// </summary>
+    [DebuggerDisplay("{DebuggerDisplay,nq}")]
+    public class UpdatePreReceiveEnvironment
+    {
+        /// <summary>
+        /// The name of the environment as displayed in the UI.
+        /// </summary>
+        public string Name { get; set; }
+
+        /// <summary>
+        /// URL to the tarball that will be downloaded and extracted.
+        /// </summary>
+        public string ImageUrl { get; set; }
+
+        internal string DebuggerDisplay
+        {
+            get { return string.Format(CultureInfo.InvariantCulture, "Name: {0} ImageUrl: {1}", Name, ImageUrl); }
+        }
+    }
+}