Feature pingurl

Hello (again),

I have added functionality to ping a URL when the tracker logs a change. It is in a similar nature to the WebHooks extension but much more basic. 

The URL if set in the preference receives a get string which has information about the changes. 

Not sure if this is a desirable feature for the main repo or not. But if it is here the patch to add it or any comment/suggestions welcomed.

Thanks,
Ross