Remove dependency on Structures_LinkedList

Use the native SplDoublyLinkedList implementation rather than the
pure-PHP Structures_LinkedList package, in theory to gain performance
and robustness bonuses, while reducing dependencies.

As SplDoublyLinkedList does not offer an add() method until PHP 5.5.0,
we need to implement our own insert methods. Still much less code than
having to rely on Structures_LinkedList.

SplDoublyLinkedList was not integrated until PHP 5.3.0, so bump the
minimum PHP version accordingly.

Signed-off-by: Dan Scott <dan@coffeecode.net>

All unit tests passing again

Still need to sort out the proper location of get/setPosition but
working well enough for now.

Signed-off-by: Dan Scott <dan@coffeecode.net>