Change end of PoW and Interest Rate

- LAST_POW_BLOCK now 2 days for testing
- COIN_YEAR_REWARD now 5%
Update Min/Max Stake Age

- Min age still 8 hours
- Max age now 30 days
- Add comments
Add nHeight as input to GetProofOfWorkReward

- Updates to main.cpp, main.h, miner.cpp
Add in varying PoW bock rewards by block number.

Updates in GetProofOfWorkReward