Add vertex filtering to AQL's TRAVERSAL[_TREE]() function

Example: 

```javascript
return TRAVERSAL_TREE(collection, edges, "collection/id", "outbound", "children", {
     filterVertices : [{isDeleted:false}]

    // representative of the default value, ignored if filterVertices not present.
    ,vertexFilterMethod : ["prune","exclude"]
});
```