Update README find_verified_user example

fix typo

* Realign `end` statements with the opening `class` statement.
* Pad JavaScript objects with spaces to match Rails styleguide and for consistency with other examples.

Share internal identifiers and message types with the JavaScript client

Allow rejecting subscriptions from the channel

Fixing subscription callbacks

Remove unnecessary space in README [ci skip]

Use ActiveSupport::Callbacks for Channel subscription callbacks.

Fix NoMethodError when using a custom Rails.logger class

small description on readme for identified_by

Send subscription confirmation from server to the client to avoid race conditions

Without this, it's very easy to send messages over a subscription even before the
redis pubsub has been fully initialized.

Now we delay calling the subscription#connected method on the client side until we
receive a subscription confirmation message from the server.

Speed up tests be calling EM.run_deferred_callbacks instead of setting a timer

* Move specific requires close to where they're needed.
* Use the private active_support/rails dep to wrap up common needs
  like eager autoload and module delegation.
* Use a single Rails engine rather than an engine and a railtie.
* Prefer `AS::JSON.encode` to `Object#to_json`.

The run_in_eventmachine test helper method is setting a 0.1 second timer to stop the event machine loop. This causes each test that requires an event machine loop to wait for 0.1 second regardless of how long the test takes to process.

This changes that to call EM.run_deferred_callbacks, which immediatly process pending actions in the event loop and then is able to exit the event loop without doing any waiting.

Before this change, running tests produced

    Finished in 2.957857s, 15.8899 runs/s, 27.7228 assertions/s.

After, the tests get

    Finished in 0.065942s, 712.7514 runs/s, 1243.5237 assertions/s.