Bug 1109641 - Disable the back/forward buttons when there is nothing to do 

Taking a stab at how communication between the Browser and the Toolbar might work. All I'm doing now is changing enabled/disabled status of the back/forward buttons, but this obviously sets up for displaying load status, stop/reload button, etc...

No hard feelings if rejected, but would love some feedback on the approach! Cheers.
Nit: We're following the style conventions listed at https://github.com/raywenderlich/swift-style-guide, which means no space before colons (here and throughout). I'll have to update the README so our conventions are more explicit :)