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

- adapter -> pubsub (re)rename internally
- Change variable names to match method names
- Add EventMachine `~> 1.0` as a runtime dependency of ActionCable
- Refactor dependency loading for adapters

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.