Turn Firebase into a module and add a Client class.

firebase-ruby currently conflicts with https://github.com/firebase/firebase-token-generator-ruby supplied by Firebase for custom authentication because in firebase-ruby Firebase is a class but in firebase-token-generator-ruby Firebase is a module.

This commit adds Firebase as a module to firebase-ruby and turns the existing code into a Client class so that you have...

```
Firebase::Client.new(...)
```