v6.0.0-alpha.0

• Big BC-breaking rewrite. Changes are numerous, but the important ones are
  listed here.
• Resource.get() now returns a State object instead of just a response
  body. This object has methods to make it easier to manipulate and get
  information about the response, including links. It's also a stable,
  non-async object.
• Ketting class is now called Client. It's still exposed as Ketting
  as well for BC purposes.
• Client.getResource() has been removed. Use Client.go() instead.
• A HTTP Fetch middleware system has been added, for easier manipulation of
  requests and responses. Middlewares can be added for every request, or
  for specific origins (domains).
• All authentication settings have been removed, and reimplemented as
  fetch middlewares. They take roughly the same options, but the setup
  has changed.
• Resource.get(), Resource.put(), etc. can now all take custom headers
  and other options to manipulate the request.
• In the past you could just send a body with Resource.post(), .put(),
  .patch(). Now this body must be a wrapped in an object with at least a .body
  property. This is an annoying BC break but will allow for more flexibility
  that was previous impossible.
• Proper support for 'binary' resources.
• Hal Links will now be reserialized on put().
• Resource.link, Resource.links and Resource.hasLink has been deprecated,
  but not removed.
• Link objects are now a simple typescript type, and no longer implemented
  as a class.
• FollowerOne is now FollowPromiseOne and FollowerMany is now
  FollowPromiseMany.
• All things called Representor has been removed, and rewritten with
  a completely new API. A HAL representor is now a HalState.
• #175: Nested embedded items are now also placed in the cache.