Update to sURL

In light of the following, I've modified the sURL service, to prevent short URL's being created that point to other known short URL providers.

http://blog.flo.cx/2009/06/loops-and-stretching-with-urlshorteners

If you find a short URL that is allowed, please let me know (I think I've covered them all, but I'm prone to error just like everyone else ;o)).