function DoublyLinkedList() {
	if ( !(this instanceof DoublyLinkedList) ) {
		return new DoublyLinkedList();
	}
	this._length = 0;
	this._first = null;
	this._last = null;
	return this;
}
Doubly linked list constructor.

@constructor
@returns {DoublyLinkedList} linked list instance

@example
var list = new DoublyLinkedList();

// Add values to the list:
list.push( 'foo' ).push( 'bar' );

// Remove the last value:
var v = list.pop();
// returns 'bar'

// Add a new value to the list:
list.push( 'beep' );

// Remove the first value:
v = list.shift();
// returns 'foo'