added wif, address, and bip32-prefix values for BlackCoin, Darkcoin, Megacoin

  BC BlackCoin mainnet : bcpv/bcpb (priv/pub)
  DRK Darkcoin mainnet : drkv/drkp
  MEC Megacoin mainnet : mecv/mecp

- changed Dogecoin mainnet bip32 prefix to dogv/dogp

- Bitcoin, Litecoin, Viacoin bip32 prefixes unchanged

- suggestion for alt-coin bip32 prefix standardization

  use first three characters of commonly accepted trading symbol with
  suffixes v and p for private and public respectively.
  two character symbols use suffixes pv and pb.
  one character symbols use suffixes pub and prv.

  testnet addresses are to use all capital letters (not yet implemented)