- do not defragment if last packets are missing
  (problem was in both defrag() and defragment())
- defragment() now puts a defragmented packet at the place of
  the latest fragment in the original capture instead of
  putting it in place of the last fragment of the datagram.

- Packet.__getitem__() raises an IndexError if asked layer is not found
- Added Packet.__setitem__() to replace a layer by another
  ex: del(pkt[Raw])
- Added Packet.__delitem__() to remove a layer and its sublayers
  ex: pkt[Raw] = Raw(load="another payload")