new Terracoin network parameters.

 - SHARE_PERIOD changed from 30 to 60 seconds. Should give more time to
   miners to actually work on jobs.
 - SPREAD changed from 15 to 3. One share will pay over a smaller amount
   of blocks but should reduce share difficulty.
 - TARGET_LOOKBEHIND updated according to the new SHARE_PERIOD value,
   also slightly decreased so the overall share diff should adjust
   faster according to effective network  hashrate
 - IDENTIFIER & PREFIX values updated to prevent confusion between
   different networks.

 - when combining new SHARE_PERIOD & new SPREAD values, average share
   difficulty should be lowered (roughly * 0.4 ?)