Added "poolServer.fixedDiff" option in /config.json

Set difficulty on miner client side by passing <address> param with .<difficulty> postfix
For example: minerd -u 4AsBy39rpUMTmgTUARGq2bFQWhDhdQNekK5v4uaLU699NPAnx9CubEJ82AkvD5ScoAZNYRwBxybayainhyThHAZWCdKmPYn.5000