Errors with larger responses

When attempting to run pointdns.records.list on a zone that about 450 records, I get the following:
```
SyntaxError: Unexpected end of input
    at Object.parse (native)
    at IncomingMessage.<anonymous> (/home/controller/app/node_modules/pointdns/pointdns.js:130:21)
    at IncomingMessage.EventEmitter.emit (events.js:95:17)
    at IncomingMessage.<anonymous> (_stream_readable.js:746:14)
    at IncomingMessage.EventEmitter.emit (events.js:92:17)
    at emitReadable_ (_stream_readable.js:408:10)
    at emitReadable (_stream_readable.js:404:5)
    at readableAddChunk (_stream_readable.js:165:9)
    at IncomingMessage.Readable.push (_stream_readable.js:127:10)
    at HTTPParser.parserOnBody [as onBody] (http.js:142:22)
```
This method works perfectly fine on smaller zones however.

It's also rather common practice with node.js to have callback functions as the final argument to any method. This isn't so much an issue, but a norm that the majority have come to expect.

I've submitted a pull request with some changes to the request to handle chunking of data on large responses. I haven't addressed any other issues, but I'd be happy to do so if you're interested.