Implement -changeaddres command line/config parameter and
getchangeaddress/setchangeaddress RPC calls.

Let's specific address for sending all change to be set. This would
prevent wallet bloat, as by default new address (public/private keypair)
is created each time there is change -- virtually at every 'send'.
Unstuck PoW by preventing negative value of the expected difficulty
(bnTarget and bnNew) and introducing block time pacing for PoW as well.
Should handle the time warp attacks.
Update version to 2.1
Set the PoW restart block.
Adjust PoW unstuck logic.
Implement unified pacing for both PoW and PoS.
Set version to 2.0.4.
Rearange pacing code.
Merge pull request #12 from danbi/master

Version 2.0.4