DoublyLinkedList.Insert() forgets dealing with element.prev

[masknu]

In order to construct a "doubly" linked list while inserting, new element and old element's prev pointer have to get updated.
Solution:
In doublylinkedlist.go,

• Line 262, add:

• newElement.prev = beforeElement
  

• Line 266, add:

• oldNextElement.prev = beforeElement
  

• Line 271, add:

• newElement.prev = beforeElement
  

• Line 274, add:

• oldNextElement.prev = beforeElement
  

[emirpasic]

@masknu Thanks, good catch! It's been fixed now in #52