Fixed bug where if result is integer and zero an object is returned.

This bug manifested when you called `getBalance` and the resulting
balance was zero. Instead of a return value of zero, you would get
the response JSON object. This is because

    fn(null, data.result || data)

will ignore data.result if data.result === 0.
Merge pull request #4 from brotchie/master

Fixed bug where if result is integer and zero an object is returned.