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
freeze time in tests
better syncing/getblocks behaviour
port BloomFilter from python-bitcoinlib
implement SPV storage backend and network logic

The backend is based on LevelDB, and stores only block headers and transactions it has been
told to watch for. If it is used, the node will set a bloom filter on all connections,
and receive filtered MerkleBlock's, stripped of most transactions we don't need.
Can sync the chain from network or import from local data files, and notify listening
clients about received transaction outputs.
cleanup handling of different modes in node/storage
end-to-end integration tests for spv mode (WIP)