feat(tooltip): update position dynamically

Reposition on each digest, this ensures that the tooltip is always positioned
correctly no matter how its content changes.

Fixes angular-ui#96
Fixes angular-ui#1109
Closes angular-ui#2816

Author: chrisirhc

Diff for: src/tooltip/tooltip.js

@@ -122,6 +122,7 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
             var ttScope = scope.$new(true);
 
             var positionTooltip = function () {
+              if (!tooltip) { return; }
 
               var ttPosition = $position.positionElements(element, tooltip, ttScope.placement, appendToBody);
               ttPosition.top += 'px';
@@ -237,6 +238,10 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
                   element.after( tooltip );
                 }
               });
+
+              tooltipLinkedScope.$watch(function () {
+                $timeout(positionTooltip, 0, false);
+              });
             }
 
             function removeTooltip() {