Changed base58 prefixes of secret key to match vanitygen convention
Changed base58 prefixes of secret key to match vanitygen convention

To be compatible with vanitygen (and others),
the prefix of SECRET_KEY should be the prefix of PUBKEY_ADDRESS + 128.
This also has the benefit of making it unambiguous that an address
is intended for use with Ribbitcoin, if user encounters
base58 secret keys (such as when redeeming a paper wallet).

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