Fix incidental hidding of the moved element.

If mouseup is followed by mousedown, without mousemove,
and there is a custom helper,
the moved element was left hidden after mouseup.

The fix makes sure that if move was not executed, the class sv-visibility-hidden is removed from the  target element.
You can observe the bug by clicking on a row from [Custom Helpers demo](http://kamilkp.github.io/angular-sortable-view/#?tab=5). After click the row disappears.