Updates to support GuzzleHTTP 5 and RingPHP handlers

Does anyone care to review?

External API hasn't changed any more than Guzzle 4-5 options have done.
This means that Adapter/Parallel Adapters are no longer supported, which resulted in removing
 - `Adapter/BatchAdapter`
 - `Subscriber/BatchSubscriber.php`
 - `Subscriber/RequestSubscriber.php`

These have been replaced by wrapping up request bodies into a single request inline.
I'm going to draw particular attention to this bit. As the comment says it's pretty lame but I can't see a more solid way around it :see_no_evil: