fix(flowDragEnter): wrong triggering of flow-drag-leave

You can reproduce the issue here: http://flowjs.github.io/ng-flow/
Drag a file into the browser. The drop area will become green. Now don't drop the file, but drag it over some links and other elements to generate `dragleave` events. You'll notice that the drop area isn't green all the time, it blinks.
Merge pull request #94 from thorn0/patch-1

fix(flowDragEnter): wrong triggering of flow-drag-leave