* make the doubly linked list more efficient
  (both memory and speed when accessing individual elements)

* add configuration options, e.g.:

   - Reverse        => new elements are inserted in front

   - AppendExisting => storing values to existing keys will
                       append (or prepend in case of Reverse)
                       this key

* add more compatibility with Tie::IxHash
  (i.e. push, pop, shift, unshift, splice, ...)

* add more tests

* add more documentation