Features/resize scrollbar after inner content updates

This feature proposes to resize the vertical scrollbar whenever the height of the ng-cells table changes and a $scope.$digest() is executed.

Note that it depends on PR #16.

**How does it work?**
- on the `ngcScrollbar` directive, we add a new `$watch` to monitor changes of height of the table body
- when a height change is detected, we schedule a scrollbar refresh in `10ms`