more iterator methods

- current() returns the slice at which the cursor of the iterator
currently points
- hasPrevious() checks if there is a previous slice to the current one
and puts the cursor of the irterator on it if there is.
- hasNext() works as hasPrevious()
Added pie.setBackground(colorString)

- possible to change the color of the ppie-background
- not able to handle images so far
Added reversedIterator

- new ReversedIterator
- pie.reversedIterator()