PPCoin: v0.4 protocol change
  - stake modifier protocol adjustment for possible future p2pool support
  - stake modifier protocol bug fix to maintain fixed generation interval
  - network protocol version updated
  - upgrade timestamp recorded in txdb and message for late upgrades
Upgrade requirement:
  Mandatory.
  Must upgrade before May 5, 2014.
  Testnet upgrade before March 24, 2014.
  Redownload of blockchain is not required for the upgrade.
  Late upgrades close to or after deadline require redownload of blockchain.
PPCoin: add hardened checkpoint for v0.4.0 release
PPCoin: Update client version to 0.4.0