Adding support for non-blocking responses and some minor API cleanup.

* Added support for non-blocking responses based on `guzzlehttp/guzzle-ring`.
* Added a public API for creating a default HTTP adapter.
* Updated the redirect plugin to be non-blocking so that redirects are sent
  concurrently. Other plugins like this can now be updated to be non-blocking.
* Added a "progress" event so that you can get upload and download progress
  events.
* Added `GuzzleHttp\Pool` which implements FutureInterface and transfers
  requests concurrently using a capped pool size as efficiently as possible.
* Added `hasListeners()` to EmitterInterface.
* Removed `GuzzleHttp\ClientInterface::sendAll` and marked
  `GuzzleHttp\Client::sendAll` as deprecated (it's still there, just not the
  recommended way).

The breaking changes in this release are relatively minor. The biggest thing to
look out for is that request and response objects no longer implement fluent
interfaces.

* Removed the fluent interfaces (i.e., ``return $this``) from requests,
  responses, ``GuzzleHttp\Collection``, ``GuzzleHttp\Url``,
  ``GuzzleHttp\Query``, ``GuzzleHttp\Post\PostBody``, and
  ``GuzzleHttp\Cookie\SetCookie``. This blog post provides a good outline of
  why I did this: http://ocramius.github.io/blog/fluent-interfaces-are-evil/.
  This also makes the Guzzle message interfaces compatible with the current
  PSR-7 message proposal.
* Removed "functions.php", so that Guzzle is truly PSR-4 compliant. Except
  for the HTTP request functions from function.php, these functions are now
  implemented in `GuzzleHttp\Utils` using camelCase. `GuzzleHttp\json_decode`
  moved to `GuzzleHttp\Utils::jsonDecode`. `GuzzleHttp\get_path` moved to
  `GuzzleHttp\Utils::getPath`. `GuzzleHttp\set_path` moved to
  `GuzzleHttp\Utils::setPath`. `GuzzleHttp\batch` should now be
  `GuzzleHttp\Pool::batch`, which returns a bjectStorage`. Using functions.php
  caused problems for many users: they aren't PSR-4 compliant, require an
  explicit include, and needed an if-guard to ensure that the functions are not
  declared multiple times.
* Rewrote adapter layer.
    * Removing all classes from `GuzzleHttp\Adapter`, these are now
      implemented as callables that are stored in `GuzzleHttp\Ring\Client`.
    * Removed the concept of "parallel adapters". Sending requests serially or
      concurrently is now handled using a single adapter.
    * Moved `GuzzleHttp\Adapter\Transaction` to `GuzzleHttp\Transaction`. The
      Transaction object now exposes the request, response, and client as public
      properties. The getters and setters have been removed.
* Removed the "headers" event. This event was only useful for changing the
  body a response once the headers of the response were known. You can implement
  a similar behavior in a number of ways. One example might be to use a
  FnStream that has access to the transaction being sent. For example, when the
  first byte is written, you could check if the response headers match your
  expectations, and if so, change the actual stream body that is being
  written to.
* Removed the `asArray` parameter from
  `GuzzleHttp\Message\MessageInterface::getHeader`. If you want to get a header
  value as an array, then use the newly added ``getHeaderAsArray()`` method of
  ``MessageInterface``. This change makes the Guzzle interfaces compatible with
  the PSR-7 interfaces.
* ``GuzzleHttp\Message\MessageFactory`` no longer allows subclasses to add
  custom request options using double-dispatch (this was an implementation
  detail). Instead, you should now provide an associative array to the
  constructor which is a mapping of the request option name mapping to a
  function that applies the option value to a request.
* Removed the concept of "throwImmediately" from exceptions and error events.
  This control mechanism was used to stop a transfer of concurrent requests
  from completing. This can now be handled by throwing the exception or by
  cancelling a pool of requests or each outstanding future request individually.
* Updated to "GuzzleHttp\Streams" 3.0.
    * `GuzzleHttp\Stream\StreamInterface::getContents()` no longer accepts a
      `maxLen` parameter. This update makes the Guzzle streams project
      compatible with the current PSR-7 proposal.
    * ``GuzzleHttp\Stream\Stream::__construct``,
      ``GuzzleHttp\Stream\Stream::factory``, and
      ``GuzzleHttp\Stream\Utils::create`` no longer accept a size in the second
      argument. They now accept an associative array of options, including the
      "size" key and "metadata" key which can be used to provide custom metadata.
