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
Can we use the "ofObject" param to tell which WebView sent this update? If so, we should make sure we only update the toolbar if the progress change is from the selected WebView.