Error correction module

$Id$

  This module provides the error correction functionality by sending
and receiving acknowledgement packets. 
This module keeps an array of structures decscribing messages already
sent. There are two options:
1. We have a broadcast message. In this case we go the traditional way.
2. We have a message to be sent to particular mote. In this case we
put the pessage (neater it's pointer) to the array and wait for an ack 
message from the receiver. If ACK message comes we remove message from 
the array and do sendDone.
  We also have the timer counting timeouts. Each time timer ticks we pass
through the array and do "if (--timeout <= 0) message sending failed"
if there are no messages we stop the timer. We start it on message sending.

-end-
