Peakle is a tiny abstraction around lists that makes them more walkable.

Peakle's constructor takes only one parameter, a list of values.

	var peakled = new Peakle([ 1, 2, 3 ]);

By default, the current position is always at the first element. If the
initial list is empty, then the current position is 'null'.

Properties:

	* length: returns a number of elements in the underlying list.
	* current: returns a value of the current element.

Methods:

	* next(): Moves to the next element and returns its value.
	* prev(): Moves to the previous element and returns its value.
	* peak(adv): Returns next element's value without moving its pointer.
	  This method accepts an optional numeric parameter. Use it if you
	  want to peak further.
	* move(index): Moves to the specified element.

All methods return 'null' if they can't return a value.

Example:

	var peakled = new Peakle([ 1, 2, 3 ]);

	peakled.current;  // 1
	peakled.peak();   // 2
	peakled.peak(2);  // 3

	peakled.next();   // 2
	peakled.peak();   // 3
	peakled.peak(2);  // null
	peakled.peak(-1); // 1

	peakled.prev();   // 1
	peakled.move(2);  // 3
