This change makes the DelegateService available on the ionic namespace.
It is useful so external directives can follow the delegate pattern set
by the framework itself.
Previously the showBack property was setting if the back button should
or should not be enabled, and it was also used if the back button
should be hidden or not for the view. Changed it so there are now two
concepts, where showBack is visual only, and enableBack determines if
it should show according to $ionicHistory and navigation info.
