ICallbackResponseBuilder + added more unit-tests (#101)

* Callback

* Add more tests

src/WireMock.Net/ResponseProviders/IResponseProvider.cs

@@ -0,0 +1,18 @@
+using System.Threading.Tasks;
+using JetBrains.Annotations;
+
+namespace WireMock.ResponseProviders
+{
+    /// <summary>
+    /// The Response Provider interface.
+    /// </summary>
+    public interface IResponseProvider
+    {
+        /// <summary>
+        /// The provide response.
+        /// </summary>
+        /// <param name="requestMessage">The request.</param>
+        /// <returns>The <see cref="ResponseMessage"/>.</returns>
+        Task<ResponseMessage> ProvideResponseAsync([NotNull] RequestMessage requestMessage);
+    }
+}