Added support to run in medium trust environments

These changes I have made allows mongocsharpdriver to run in Medium Trust environments like Rackspace Cloudsites.

The key problem that was preventing medium trust was TransformFinalBlock method.

Now, for medium trust environments, when declaring your MongoClient, you can add the optional parameter of 'true' to not use secure strings. This is similar to AWS's c# library for anyone who is familiar with it.

            client = new MongoClient(connectionString, true);