ICallbackResponseBuilder + added more unit-tests (#101)

* Callback

* Add more tests

src/WireMock.Net/ResponseBuilders/ICallbackResponseBuilder.cs

@@ -0,0 +1,19 @@
+using System;
+using JetBrains.Annotations;
+using WireMock.ResponseProviders;
+
+namespace WireMock.ResponseBuilders
+{
+    /// <summary>
+    /// The CallbackResponseBuilder interface.
+    /// </summary>
+    public interface ICallbackResponseBuilder : IResponseProvider
+    {
+        /// <summary>
+        /// The callback builder
+        /// </summary>
+        /// <returns>The <see cref="IResponseBuilder"/>.</returns>
+        [PublicAPI]
+        IResponseBuilder WithCallback([NotNull] Func<RequestMessage, ResponseMessage> callbackHandler);
+    }
+}