This commit adds ActiveSupport::Notifications instrumentation hooks
and a LogSuscriber to ActionCable::Channel::Base.

* Fix typos/grammar errors
* Make capitalization/naming consistent

When running the ActionCable server in development I get a lot of output
in my logs, this commit sets a maximum length of 300 characters for a
broadcast log message.

This PR checks all active Action Cable documentation for typos and other
fixes. It aims to make sure that when Rails 5 is released, that the
Action Cable docs are up to snuff with the other documentation included
with Rails.

[ci skip]

* Properly indent code sample in ActionCable::Channel::Streams
* Add a doc comment for #stop_all_streams
* Reformat + add <tt> blocks around code references in ActionCable::Base docs
* Clarify and a little better grammar on ActionCable::RemoteConnections
* Correct indentation and clean up ActionCable::Server::Broadcasting code sample

Warnings coming from code and test are removed

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.

* 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`.

 * Rely on AS::Callbacks for callback handling.
 * Add before_subscribe, after_subscribe, before_unsubscribe and
   after_unsubscribe convenience methods
 * alias on_subscribe and on_unsubscribe to after_subscribe and
   after_unsubscribe, respectively.
 * Remove `subscribed` and `unsubscribed` from the callback chain:
   these methods are now executed as the subject of the callbacks.
 * Update portions of ActionCable to use the more specific
   callback names.

Fixes #14

We have the remote connections to immediately cut a connection when
someone has been kicked off. Let’s lean on that for now.