Don't move specific elements

[jakobmaier]

Feature request:
In my current use case, it should not be possible to drag the last element produced by ng-repeat and it should also not be possible to move another element below the last one.

The first issue can be solved by hiding the sv-handle with ng-show, but it can't be avoided to drag other elements below the last one.

To achieve this, the sv-on-start and sv-on-stop functions could be modified to return false if the reorder is not allowed to happen.