scripts: add dataOutput convenience function

As the title says.

The script is very simple to build, so I'm not 100% whether this is necessary.

Example 1 (new method):
``` javascript
    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):
``` javascript
    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?