Bug that the containerSize is not known yet.

In my case the mb-scrollbar directive is inited before the size is known. Because the height is calculated afterwards with another directive. At the start the `containerSize` is `0`. And the `containerSize` is not updated in the `recalculate()` In this fix `containerSize` is updated after every `recalculate()`.

Also added a `scrollTo` method that can be called on the `mbService`.