Allow a callback to be passed for the cancelCallback argument to firebaseRef.on()

Not sure how to add a test to your suite for this, but have checked it at least works locally in react-hn against the Hacker News API, which errors when attempting to load delayed comments
Added new argument to _bind() test
Merge pull request #24 from insin/cancelCallback

Allow a callback to be passed for the cancelCallback argument to firebaseRef.on()