Change pubkey addresses for multisig addresses and testnet.

I propose to update public key versions from the bitcoin defaults to darkcoin uniques. To avoid confusion:
- change script public key address prefex from '3' (like bitcoin) to '7'
- change testnet public key address prefix from 'm/n' (like bitcoin) to 'x/y'
- change testnet script public key address prefix from '2' (like bitcoin) to '8/9'

Impact is to be discussed. The private key versions are untouched in this commit, so any fund is still valid and redeemable.