Support nonce override in sign()

[0xJem]

It would be useful to be able to specify a nonce, so that multisig batches can be queued.

e.g. There is an existing batch with nonce 10, but because it has not been executed yet, getNonce() returns 10. Submitting a proposal would create a batch with the same nonce of 10.

[0xJem]

Ah this isn't necessary - it can be achieved through proposeTransaction() providing the params using the ExecTransactionParams struct.

[0xJem]

Actually this is possible, but only if sign() accepts a nonce parameter.

[0xJem]

Closed in #19