def signed(pvt, id)
      t = clone
      t.sign = Signature.new.sign(pvt, id, self)
      t
    end
def signed(pvt, id)
      t = clone
      t.sign = Signature.new.sign(pvt, id, self)
      t
    end
Sign the transaction and add RSA signature to it
 +pvt+:: The private RSA key of the paying wallet
 +id+:: Paying wallet ID