Basic SDK functionality

[luismfonseca]

This PR brings basic SDK functionality:

• Token registration (with refresh token support)
• Subscribe and Unsubscribe
• Basic message handling (ignoring pusher blank notification)
• Minimal linter to warn about bad instances ids

There are a few things I'm still trying to figure out here:

• What permissions belong to the library and not to the example app
• How to display notifications right off the bat

[luismfonseca]

Thanks for the review @zmarkan and @lukabratos.

I've incorporated your feedback and I'm leaving some other changes to other PRs.