Add callbacks to respond back o UI when registering

[paulincai]

e.g. I want to subscribe to Notifications. My browser blocks notification. activitree:push can sense this and return this message to UI.

Logic: Press "Subscribe" -> receive message: Please allow your browser to receive notifications.

[paulincai]

Implemented scenario:
* User Signs-in on this device. The user in Meteor has opted in the past to receive Push notifications from this app.
* He is supposed to receive Push on whichever devices is enabled by the user to provide these Push Notifications.
* I try to get a token. If I get one (browser gives it to me) I save this userId in Mongo associated with this token. If there was already a user on the token I overwrite the old user.
* If the browser blocks me, tell the user he is set to receive Push notifications but the browser blocks that.
* If the user is not set to receive Push just disregard everything. (do not call a getToken)
* When the user logs out the app from a browser, unsubscribe the user from notifications in that browser.

[paulincai]

This is now completed. Callbacks added will just need to add the documentation and a React component as example.