Update difficulty retargeting to i0coin-0.8.x branch.
Update difficulty retargeting to i0coin-0.8.x branch.

See also last commit, there was apparently another fork in I0coin that
also affected the difficulty retargeting algorithm.

domob1812/i0coin,domob1812/i0coin,domob1812/i0coin,domob1812/i0coin,domob1812/i0coin,domob1812/i0coin