Added unit tests (all passing unfortunately)

UI improvements
Write mode still doesn't work

ModernKeePassLib.Test/Cryptography/HmacOtpTests.cs

@@ -0,0 +1,36 @@
+using NUnit.Framework;
+using System;
+using System.Text;
+
+#if KeePassLib
+using KeePassLib.Cryptography;
+#else
+using ModernKeePassLibPCL.Cryptography;
+#endif
+
+namespace ModernKeePassLib.Test.Shared.Cryptography
+{
+  [TestFixture ()]
+  public class HmacOtpTests
+  {
+    // Using the test case from Appendix D of RFC 4226
+
+    const string secret = "12345678901234567890";
+    static readonly string[] expectedHOTP = new string[] {
+      "755224", "287082", "359152", "969429", "338314",
+      "254676", "287922", "162583", "399871", "520489"
+    };
+
+    [Test ()]
+    public void TestGenerate ()
+    {
+      var secretBytes = Encoding.UTF8.GetBytes (secret);
+
+      for (ulong i = 0; i < 10; i++) {
+        var hotp = HmacOtp.Generate (secretBytes, i, 6, false, -1);
+        Assert.That (hotp, Is.EqualTo (expectedHOTP[i]));
+      }
+    }
+  }
+}
+