Fix high CPU usage with Safari browsers

[choco]

The high CPU usage when using Shifty with Safari browser comes from:

• poor scripting bindings in safari (which make every call super expensive)
• high amount of call (because the notifications we listen to fire too many times)

To address this we can change the notification we listen too, specifically:

• loadComplete is triggered when a page finishes loading (to handle url change when user is navigating)
• focusedTabChanges when changing between browser tabs
  We still need to keep uiElementDestroyed to account for when a user leaves a full screen video window.

We also only use the AX api to obtain the URL, which is faster and more lightweight.
I've been using those changes for almost two years so I wouldn't worry about the AX hierarchy stability!

This addresses #101 and #90