squashed commits from #27
key version check always always relies on the network property

[#27]
HD wallet versions: Bitcoin -> prod, BitcoinTest -> testnet
move address versions into network

Also change address type name:
address_types -> addressVersion
p2sh_types -> p2shVersion
prod -> mainnet
HDWallet prod -> mainnet
throws exception if specified network is unknown

also move network type tests to hdwallet
pull out hdwallet network specific values
Merge pull request #28 from bitcoinjs/hd-testnet-support

HD testnet support (#27) + unify network names