Add IFileSystemHandler to support Azure for StaticMapping location (#180)

* wip

* CustomStaticMappingFileHandler

* Add unit-tests

* Tests

* IFileSystemHandler

* version

test/WireMock.Net.Tests/Util/FileHelperTests.cs

@@ -0,0 +1,44 @@
+using System;
+using System.IO;
+using Moq;
+using NFluent;
+using WireMock.Handlers;
+using WireMock.Util;
+using Xunit;
+
+namespace WireMock.Net.Tests.Util
+{
+    public class FileHelperTests
+    {
+        [Fact]
+        public void FileHelper_ReadAllTextWithRetryAndDelay()
+        {
+            // Assign
+            var _staticMappingHandlerMock = new Mock<IFileSystemHandler>();
+            _staticMappingHandlerMock.Setup(m => m.ReadMappingFile(It.IsAny<string>())).Returns("text");
+
+            // Act
+            string result = FileHelper.ReadAllTextWithRetryAndDelay(_staticMappingHandlerMock.Object, @"c:\temp");
+
+            // Assert
+            Check.That(result).Equals("text");
+
+            // Verify
+            _staticMappingHandlerMock.Verify(m => m.ReadMappingFile(@"c:\temp"), Times.Once);
+        }
+
+        [Fact]
+        public void FileHelper_ReadAllTextWithRetryAndDelay_Throws()
+        {
+            // Assign
+            var _staticMappingHandlerMock = new Mock<IFileSystemHandler>();
+            _staticMappingHandlerMock.Setup(m => m.ReadMappingFile(It.IsAny<string>())).Throws<NotSupportedException>();
+
+            // Act
+            Check.ThatCode(() => FileHelper.ReadAllTextWithRetryAndDelay(_staticMappingHandlerMock.Object, @"c:\temp")).Throws<IOException>();
+
+            // Verify
+            _staticMappingHandlerMock.Verify(m => m.ReadMappingFile(@"c:\temp"), Times.Exactly(3));
+        }
+    }
+}