WARNING: API change. Work on Packet.__getitem__(), __setitem__() and __delitem__()

- 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")