feat(callbacks): Add onComplete callbacks

[davideast]

Closes #16.

The Firebase SDK allows you to pass a callback to ensure when the operation has succeeded on the server. Querybase now mirrors this API.

const dbRef = firebase.database().ref('things');
const queryRef = querybase.ref(dbRef, ['a', 'b']);
queryRef.push('thing', error => {
  console.log('completed!');
});

[coveralls]

Coverage remained the same at 99.405% when pulling 8c37b76 on callbacks into 58b9678 on master.