Fixed issue #2309 wit new functions to get traversed elements

We'd need 3 new functions:

traversedVertex, to return the traversed vertex
traversedEdge, to return the traversed edge
traversedElement, to return any of the traversed element
All functions should get 2 params:

index, as starting index. negative numbers means from the end, positive
from the beginning of the stack. Example 0 = absolute first traversed
vertex, -1 = relative last one
items, as optional, by default is 1, but specifying >1 collects the
elements in a collection.
Example:

select $path, traversedVertex(-1) from ( traverse out() from #9:1 while
$depth < 10 )