Fixed and refactored the db.Reference.listen() API

[hiranya911]

Continuation of #183

• KeepAuthSession now extends from the AuthorizedSession. This allows the listen() API to properly authorize outgoing requests.
• Added db.Event type. The callback gets fired with instances of this class.
• Updated documentation and changelog.
• Cleaned up the code and added more unit tests.

[hiranya911]

@the-c0d3r please take a look.

[the-c0d3r]

@hiranya911 I have looked at the changes. It looks good, much cleaner than what I wrote. But the problem you mentioned in #183 I do not see the problem. But I have been using the SDK with the authentication json file. So that may be obscuring the problem.

[hiranya911]

@the-c0d3r You probably have your database security rules set to public, allowing unauthenticated access to the data. I encountered the issue when testing with a service account json file.