New tests for signMultiSigUTXO (previously SignMultiSigUTXO)

signMultiSigUTXO now also calls validateSigScript() to ensure the sigscript
it generates will execute later on.

New SignTx() function which signs all inputs of a given tx.

Plus removal of createTxWithInputAmounts, which is no longer needed as we
have the more flexible createDecoratedTx.

And last but not least, some fixes for issues pointed out by go vet.
It's hard for me to see that i's really a pubkey hash - maybe do an assertion on the type (AddressPubKeyHash) of the addr that's returned, so the reader can be absolutely sure that it's testing the right thing?