Remove Guzzle in flavor of PSR-18

[ker0x]

As soon as the PSR-18 will be accepted, we need to:

• remove guzzlehttp/guzzle from required package
• add psr/http-client instead
• rework API classes to take into account ClientInterface

[misantron]

I can make it if you want

[chris001]

@misantron @ker0x PSR-18 was approved in November, 2018! https://www.php-fig.org/blog/2018/11/psr-18-the-php-standard-for-http-clients/