new scriptpubkey_to_address()

@adamkrellenstein, to avoid circular import I had to move `pubkey_to_pubkeyhash()`, `get_asm()`, `get_checksig()` and `get_checkmultisig()`   in  `util.py`. Do you think we should move all this functions in `bitcoin.py` or move `pubkeyhash_to_pubkey()` in `bitcoin.py`.