This service hook adds an object to a given Firebase for every commit.

The commit will be appended, in-order, to the URL provided in the 'firebase'
configuration option. This is the equivalent of calling:

    new Firebase(firebase).push(commit);

from the JS library.

The provided URL must point to a valid Firebase reference, and must be prefixed
with https://

If you have secured your Firebase with security rules that prevent anonymous
writes, you may provide the secret to your firebase in the configuration
options.
