added key, address, and bip32 prefixes for BlackCoin, Darkcoin, Megacoin, Dogecoin

selections 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.

lowercase prefix for mainnet, testnet addresses are to use all capital letters (not implemented)

minor formatting
bitcoin, litecoin, and viacoin prefixes left unchanged
good idea, will squash a new commit tomorrow with all suggested changes.