feat($alert): add "remove" method to remove single alert
feat(alertDirective): separate alerts and alert directive
feat(alertDirective): optionally invalidate alert

Expire and hide alert. The time can be set in ms to alertsTTL (this feature is disabled if alertsTTL is false or negative)
refactor($alert): add scope as Alert dependency
docs(demo): add bootstrap CSS
feat($alert): add removeAll method to remove all present alerts
docs(alertDirective): add example