Gonna inherit from it on two subclasses: "P2SH Input" and "P2PKH Input"
They'll have a `addSignature` method, to deal with new signatures,
a `clearSignatures`, in case the outputs change and the signatures get
invalidated, `countMissingSignatures`, and a reference to the UTXO
they'll be spending, so all logic regarding "p2sh mapping" gets to
be in here and the Transaction class is not polluted.
Esteban Ordano