Add new funcs for creating payment scripts.

This commit adds two new functions named PayToScriptHashScript and
PayToAddrScript.  The first one creates and returns a public-key script
which pays to the provided script hash and conforms to BIP0016.

The second function takes the new btcutil.Address interface type and
returns an appropriate script to pay to the address type in the interface.
It currently works for btcutil.AddressPubKeyHash and
btcutil.AddressScriptHash.