fix(tooltip): Fixed Angular 1.3.1 $observe bug

Angular 1.3.1 changes the behaviour of the $observe function in that it no longer fires when an attribute is undefined (https://github.com/angular/angular.js/commit/531a8de72c439d8ddd064874bf364c00cedabb11). The tooltip module relies on this behaviour to set up tooltips . This PR checks for undefined attributes and fires the relevant functions when $observe will not. This fixes #142.