fix(tooltip) Tooltip is not re positioned after changing its text content

[hadrienl]

Hi, if you display a tooltip with an expression and this expression change while the tooltip is displayed, then it's not redimensionned and re positionned like you can see here : http://plnkr.co/edit/6ZdI7F8YrwAaM4bUzboI?p=preview

[hadrienl]

I've made a working workaround :

attrs.$observe('tooltip', function (val) {
  if (tooltip) {
    removeTooltip();
    $timeout(show);
  }
});

demo here : http://plnkr.co/edit/6D1Vj2oWuGQIaOf83cGR?p=preview

[hadrienl]

PR : #2816

[pkozlowski-opensource]

This is duplicate of #96