Adding support for push notifications to all devices by one request

[grey-dev-0]

The changes proposed in this request is for supporting the new Topic push notification provided by Google, which allows the developers to send push notifications to all Android devices with one request instead of sending them in batches including multiple registration IDs.

[juliangut]

Fully supporting the "topic" concept by allowing setting the 'to' and 'condition' parameter seems a better solution

From Firebase documentation:

• About 'to' parameter: "The value must be a registration token, notification key, or topic. Do not set this field when sending to multiple topics. See condition."
• About 'registration_ids' parameter: "Use this parameter only for multicast messaging, not for single recipients. Multicast messages (sending to more than 1 registration tokens) are allowed using HTTP JSON format only."

If only one device token is added to the message then message formatting (->toJson()) should not return 'registration_ids' key with only that one token but a 'to' key

In order to support /topics/global topic it could be a constant in the class to be used in ->setTo()

[mwillbanks]

PR #31 has a far better implementation on this just pending an issue with the condition area.