Bug 1109647 Add progress loading

Here is a simple implementation of the browser loading progress. Not sure if later down the road we should be trying to keep only one KVO each time we switch tabs? Anyways I hope this points us in the right direction.

As always I appreciate the team's thoughts.

Thanks
@mfinkle 
I just made a slight update checking the ofObject to ensure that the selected tab has the right web view prior to updating the progress bar. Let me know if that is what you are looking for.

Thanks for suggestion!