added pubkeys_to_p2sh_multisig_address and tests
added redeem_script to return value in pubkeys_to_p2sh_multisig_address
use testnet url for tx relay example

so it is consistent with the rest of the transaction example
clean up address and script types

this makes for consistent naming of the address/script types everywhere,
gives us some more flexibility to introduce new types,
makes #encode/decode_address more generic and
highlights the importance of carrying the address type along the hash160,
while still preserving the old APIs for compatibility
clean up network descriptions
TxBuilder::sign_input now allows missing keys in multisig and p2sh_multisig