1. Adopting a marker interface for Guzzle exceptions.
        A. All exceptions emitted from Guzzle are now wrapped with a Guzzle namespaced exception.
        B. Guzzle\Common\GuzzleExceptionInterface was renamed to Guzzle\Common\GuzzleException
        C. Guzzle\Common\ExceptionCollection was renamed to Guzzle\Common\Exception\ExceptionCollection
    2. Using Header objects for Request and Response objects
        A. When you call $request->getHeader('X'), you will get back a Guzzle\Http\Message\Header object that contains all of the headers that case insensitively match.  This object can be cast to a string or iterated like an array.  You can pass true in the second argument to retrieve the header as a string.
        B. Removing the old Guzzle\Common\Collection based searching arguments from most of the request and response header methods.  All retrievals are case-insensitive and return Header objects.
    3. Changing the two headers added by the cache plugin to just one header with key and ttl.
    4. Changing Guzzle\Http\Message\Response::factory() to fromMessage().
    5. Removing the NullObject return value from ServiceDescriptions and instead simply returning null

New Features / enhancements:

    1. Adding Guzzle\Http\Message\AbstractMessage::addHeaders()
    2. Making it simpler to create service descriptions using a unified factory method that delegates to other factories.
    3. Better handling of ports and hosts in Guzzle\Http\Url

Note: This is a noisy diff because I'm removing trailing whitespace and adding a new line at the end of each source file.