scripts: add dataOutput convenience function

[dcousens]

As the title says.

The script is very simple to build, so I'm not 100% whether this is necessary.

Example 1 (new method):

    var tx = new bitcoin.TransactionBuilder()

    var data = new Buffer('cafedeadbeef', 'hex')
    var dataScript = bitcoin.scripts.dataOutput(data)

    tx.addInput("aa94ab02c182214f090e99a0d57021caffd0f195a81c24602b1028b130b63e31", 0)
    tx.addOutput(dataScript, 1000)
    // ...

Example 2 (without this PR):

    var tx = new bitcoin.TransactionBuilder()

    var data = new Buffer('cafedeadbeef', 'hex')
    var dataScript = bitcoin.Script.fromChunks([bitcoin.opcodes.OP_RETURN, data])

    tx.addInput("aa94ab02c182214f090e99a0d57021caffd0f195a81c24602b1028b130b63e31", 0)
    tx.addOutput(dataScript, 1000)
    // ...

Personally, the second is as clear, but, I feel like the complexity surrounding this functionality is already high enough that perhaps this PR helps?
Thoughts?

[dcousens]

ping @williamcotton

[coveralls]

Coverage increased (+0.0%) when pulling ea66ede on datascript into 27e69ba on master.

[weilu]

IMHO an example on the REAME would be sufficient, but I haven't used OP_RETURN in any prod code. I'm curious to hear from those who actually use OP_RETURN.

[dcousens]

@weilu understandably. But, at what point do you do the same for pubKeyInput (which is simply just Script.fromChunks([signature]) etc), the point is this might relinquish some of the burden of knowing how the implementation actually works by making a very simple, but none the less abstracting ... abstraction.

[williamcotton]

👍

[williamcotton]

Reading from OP_RETURN is useful as well:

var outputs = transaction.outputs;
for (var j = outputs.length - 1; j >= 0; j--) {
  var output = outputs[j];
  var scriptPubKey = otuput.scriptPubKey;
  var scriptPubKeyBuffer = new Buffer(scriptPubKey, 'hex')
  if (scriptPubKeyBuffer[0] == 106) { // OP_RETURN code is 106
    var messageBuffer = scriptPubKeyBuffer.slice(2,scriptPubKeyBuffer.length);
    var message = messageBuffer.toString();
  }
};