S7.NET Testability Updates

Removed the Controls project and existing Test projects. Added new Test
project and references to NUnit and Should projects for testing.
Extracted IPlc interface to help test projects that reference this
library. Also implemented the IDisposable interface on the Plc object so
that references to mSocket would be cleaned up appropriately when the
object is blown away.

S7.Net/Types/String.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace S7.Types
+{
+    public static class String
+    {
+        // publics
+        #region ToByteArray
+        public static byte[] ToByteArray(string value)
+        {
+            string txt = (string)value;
+            char[] ca = txt.ToCharArray();
+            byte[] bytes = new byte[txt.Length];
+            for (int cnt = 0; cnt <= ca.Length - 1; cnt++)
+                bytes[cnt] = (byte)Asc(ca[cnt].ToString());
+            return bytes;
+        }
+        #endregion
+
+        #region FromByteArray
+        public static string FromByteArray(byte[] bytes)
+        {
+            return System.Text.Encoding.ASCII.GetString(bytes);
+        }
+        #endregion
+
+        // privates
+        private static int Asc(string s)
+        {
+            byte[] b = System.Text.Encoding.ASCII.GetBytes(s);
+            if (b.Length > 0)
+                return b[0];
+            return 0;
+        }
+    }
+}