Added IMouse.cs and Mouse.cs, fixing the compilation on mono.

Source/OpenTK/Input/IMouse.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace OpenTK.Input
+{
+    interface IMouse : IInputDevice
+    {
+    }
+}

Source/OpenTK/Input/Mouse.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace OpenTK.Input
+{
+    public class Mouse : IMouse
+    {
+        #region --- IInputDevice Members ---
+
+        public string Description
+        {
+            get { throw new Exception("The method or operation is not implemented."); }
+        }
+
+        public InputDeviceType DeviceType
+        {
+            get { return InputDeviceType.Mouse; }
+        }
+
+        #endregion
+    }
+}