Guzzle 4

This PR updates the library to support Guzzle 4.0 and introduces a number of breaking changes:

* Update directory structure to PSR-4 
* Update to Guzzle 4
* Remove 5.3 
* `EwayClient::__construct` parameters changed to Guzzle 4 `GuzzleHttp\Command\Guzzke\GuzzleClient` parameters
* `Bdt\Eway\Log\MessageFormatter` is now `Bdt\Eway\Formatter`