Added some place holder classes for various modules.

Some work on the asset cache, can people please test this. including on one of the public systems so we can see if it causes problems with multiple users.

OpenSim/Region/Environment/Modules/InstantMessageModule.cs

@@ -0,0 +1,10 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace OpenSim.Region.Environment.Modules
+{
+    public class InstantMessageModule
+    {
+    }
+}