Improve behaviour of the "on" lint

[gitnik]

• The lint will from now on only handle "$on" calls, as "$watches" are automatically removed upon destroying the scope. I therefore also changed the name of the lint to match what is does.
• The lint will no longer complain about not unsubscribing from the $rootScope. While there can be cases where this is valid, most of the time it isn't
• Adapt tests
• Adapt documentation

[gitnik]

Fixes #231

[EmmanuelDemey]

Can you do your PR on the development branch? Thanks by the way ;)

[charneykaye]

Any reason not to do this?

$rootScope.$on('$destroy', $rootScope.$on(EVENTS.changeRegion, function() {
  vm.nowRegion = OpsService.getRegion();
}));

[mustela]

@gitnik any plan to work on this one?

Thanks

[briananderson1222]

Would love to see this

[ivan-zakharchuk]