Allow token ID or API key in the client constructor

[mattheworiordan]

Either should be usable in the format Ably::Realtime.new(api_key) or Ably::Realtime.new(token_id)

[kouno]

Note: Documentation in README needs to be updated too to complete this task. In particular Authentication with a token needs to show a better example.

[mattheworiordan]

See ably/ably-js@1785597...5a8ec24

[mattheworiordan]

Solved, #19