refactor(tooltip): remove observer for triggers

This change fixes the usage of the tooltip directive with AngularJS
1.3.1 when no trigger attribute is used.

BREAKING CHANGE: tooltip/popover trigger is no longer a watched
attribute.
This affects both popovers and tooltips. The triggers are now set up
once and can no longer be changed after initialization.

src/tooltip/test/tooltip.spec.js

@@ -271,7 +271,7 @@ describe('tooltip', function() {
       expect( elmScope.tt_isOpen ).toBeFalsy();
     }));
 
-    it('should not share triggers among different element instances - issue 692', inject( function ($compile) {
+    it('should only set up triggers once', inject( function ($compile) {
 
       scope.test = true;
       elmBody = angular.element(
@@ -290,10 +290,12 @@ describe('tooltip', function() {
 
       scope.$apply('test = false');
 
-      elm2.trigger('mouseenter');
+      // click trigger isn't set
+      elm2.click();
       expect( elmScope2.tt_isOpen ).toBeFalsy();
 
-      elm2.click();
+      // mouseenter trigger is still set
+      elm2.trigger('mouseenter');
       expect( elmScope2.tt_isOpen ).toBeTruthy();
     }));
   });

src/tooltip/tooltip.js

@@ -281,7 +281,8 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
               element.unbind(triggers.hide, hideTooltipBind);
             };
 
-            attrs.$observe( prefix+'Trigger', function ( val ) {
+            function prepTriggers() {
+              var val = attrs[ prefix + 'Trigger' ];
               unregisterTriggers();
 
               triggers = getTriggers( val );
@@ -292,7 +293,8 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
                 element.bind( triggers.show, showTooltipBind );
                 element.bind( triggers.hide, hideTooltipBind );
               }
-            });
+            }
+            prepTriggers();
 
             var animation = scope.$eval(attrs[prefix + 'Animation']);
             scope.tt_animation = angular.isDefined(animation) ? !!animation : options.animation;