Remove requirement for passing client to models

[shalvah-gs]

As pointed out in #65, when you need to initialize a model, you must pass a client instance, or else you'll get an error "client is not ready":

Calendly::Event.new({ uri: }).fetch # raises an error

But there's no value to this error–to avoid it, you just need to add a boilerplate client, which is easy to forget, annoying and pointless:

Calendly::Event.new({ uri: }, Calendly::Client.new).fetch

This PR gets rid of this requirement. If no client is passed, a new client instance is created. It's what the user does most times anyway.