0.9.0

• Updated to use Guzzle 6 and PSR-7.
• Event system has been replaced with a middleware system
  • Middleware at the command layer work the same as middleware from the
    HTTP layer, but work with Command and Result objects instead of
    Request and Response objects
  • The command middleware is in a separate HandlerStack instance than the
    HTTP middleware.
• Result objects are the result of executing a Command and are used to hold
  the parsed response data.
• Asynchronous code now uses the guzzlehttp/promises package instead of
  guzzlehttp/ringphp, which means that asynchronous results are implemented
  as Promises/A+ compliant Promise objects, instead of futures.
• The existing Subscribers were removed.
• The ServiceClientInterface and ServiceClient class now provide the basic
  foundation of a web service client.