Update main.cpp to switch to pure PoS quickly

Minimum coin age required for minting reduced to 3 days from 30 and coin maturity reduced to 100 from 500 to allow quick shift to proof of stake minting after genesis block.
Constants for total number of IPO shares (set to 1000000) and block height after which system switches from proof of work to proof of stake (set to 400) added.
When block height is <= 400, blocks will be proof of work, otherwise proof of stake.
The initial target interval for blocks will be 30 minutes to keep the blockchain small.