Zetacoin: Changed part of the core parameters

Genesis Block
- Coins awarded in Genesis block
- Difficulty Target
- Time
- Nonce

Difficulty
- Retarget Timespan (From 2 weeks to 2 minutes)
- Block time (From 10 minutes to 30 seconds)

Network Parameters
- Address Header ("Z")
- Private Key Header
- Port number
- Packet Magic
- Block Interval for Reward Halving

Left to do:
- Protocol Version
- ProofOfWork Limit - need to figure out what CBigNum(~uint256(0) >> 20) is represented as in compact bits
- Checkpoints (maybe easiest)
- Seed peers
- Genesis block's input scriptSig with the message following difficulty bits
-- "3 Aug 2013 - M&G - Mugabe wins Zim election with more than 60% of votes"