use new firebase admin sdk private key

[rflorence]

To maintain backwards compatibility, I kept the same auth parameter. If it's a filepath, treat it as an Admin SDK private key file. Otherwise, treat it as a database secret key.

Fixes #64, #80

[vincentwoo]

Thanks for the PR! To merge, I would like to request that we change the argument from the filepath of the auth file to the string contents of the auth file for two reasons:

1. This matches the passing scheme for the old secret.
2. This works better in environments like Heroku that typically pass secrets via environment header. In general, there are many ways to pass secrets to applications and hopefully the library can accept all of them.

[rflorence]

That sounds good to me. Done.