Allow usage with firebase-node

[tobiash]

In order to use this library for a React-Native project, I'd like to use it with cljsjs.firebase-node instead of cljsjs.firebase, since the latter has hard dependencies on the DOM, which is not available in a RN project.
Is it possible to replace the dependency on cljsjs.firebase with something configureable?

[burn2delete]

Use with node will require another protocol and API, feel free to submit a PR.